When deciding whether to scan the global run queue or not, we currently
check the configured threads number, and if it's 1 we skip the queue
since it's not supposed to be used. However when running with a master
process and multiple threads in the workers, the master will turn this
number back to 1 while some task wakeups might possibly have set bits
in the global tasks mask, thus causing active_tasks_mask to have one
bit permanently set, preventing the process from sleeping.
Instead of checking global.nbthread, let's check for the current
thread's bit in global_tasks_mask. First it will make this part of the
code more consistent, working like a test and set operation, it will
solve the issue with master+nbthread and as a bonus it will save a
lock/unlock for each scheduler call when the thread doesn't have a
task in the global run queue.
Commit 27f3fa5 ("BUG/MEDIUM: mworker: stop every tasks in the master")
used MAX_THREADS as a mask instead of MAX_THREADS_MASK to clean the
global run queue, and used rq_next (global variable) instead of next_rq.
Renamed next_rq as tmp_rq and next_wq as tmp_wq to avoid confusion.
No backport needed.
The master is not supposed to run (at the moment) any task before the
polling loop, the created tasks should be run only in the workers but in
the master they should be disabled or removed.
No backport needed.
signal_init(), init_log(), init_stream(), and init_task() all used to
only preset some values and lists. This needs to be done very early to
provide a reliable interface to all other users. The calls used to be
explicit in haproxy.c:init(). Now they're placed in initcalls at the
STG_PREPARE stage. The functions are not exported anymore.
This commit replaces the explicit pool creation that are made in
constructors with a pool registration. Not only this simplifies the
pools declaration (it can be done on a single line after the head is
declared), but it also removes references to pools from within
constructors. The only remaining create_pool() calls are those
performed in init functions after the config is parsed, so there
is no more user of potentially uninitialized pool now.
It has been the opportunity to remove no less than 12 constructors
and 6 init functions.
This patch replaces a number of __decl_hathread() followed by HA_SPIN_INIT
or HA_RWLOCK_INIT by the new __decl_spinlock() or __decl_rwlock() which
automatically registers the lock for initialization in during the STG_LOCK
init stage. A few static modifiers were lost in the process, but since they
were not essential at all it was not worth extending the API to provide such
a variant.
Right now we measure for each task the cumulated time spent waiting for
the CPU and using it. The timestamp uses a 64-bit integer to report a
nanosecond-level date. This is only enabled when "profiling.tasks" is
enabled, and consumes less than 1% extra CPU on x86_64 when enabled.
The cumulated processing time and wait time are reported in "show sess".
The task's counters are also reset when an HTTP transaction is reset
since the HTTP part pretends to restart on a fresh new stream. This
will make sure we always report correct numbers for each request in
the logs.
Currently we have per-thread arrays of trees and counts, but these
ones unfortunately share cache lines and are accessed very often. This
patch moves the task-specific stuff into a structure taking a multiple
of a cache line, and has one such per thread. Just doing this has
reduced the cache miss ratio from 19.2% to 18.7% and increased the
12-thread test performance by 3%.
It starts to become visible that we really need a process-wide per-thread
storage area that would cover more than just these parts of the tasks.
The code was arranged so that it's easy to move the pieces elsewhere if
needed.
Now we still have a main contention point with the timers in the main
wait queue, but the vast majority of the tasks are pinned to a single
thread. This patch creates a per-thread wait queue and queues a task
to the local wait queue without any locking if the task is bound to a
single thread (the current one) otherwise to the shared queue using
locking. This significantly reduces contention on the wait queue. A
test with 12 threads showed 11 ms spent in the WQ lock compared to
4.7 seconds in the same test without this change. The cache miss ratio
decreased from 19.7% to 19.2% on the 12-thread test, and its performance
increased by 1.5%.
Another indirect benefit is that the average queue size is divided
by the number of threads, which roughly removes log(nbthreads) levels
in the tree and further speeds up lookups.
The run queue is designed to perform a single tree lookup and to
use multiple passes to eb32sc_next(). The scheduler rework took a
conservative approach first but this is not needed anymore and it
increases the processing cost of process_runnable_tasks() and even
the time during which the RQ lock is held if the global queue is
heavily loaded. Let's simply move the initial lookup to the entry
of the loop like the previous scheduler used to do. This has reduced
by a factor of 5.5 the number of calls to eb32sc_lookup_get() there.
Instead of checking if nbthreads == 1, just and thread_mask with
all_threads_mask to know if we're supposed to add the task to the local or
the global runqueue.
Depending on the optimization level, gcc may complain that wake_thread()
uses an invalid array index for poller_wr_pipe[] when called from
__task_wakeup(). Normally the condition to get there never happens,
but it's simpler to ifdef out this part of the code which is only
used to wake other threads up. No backport is needed, this was brought
by the recent introduction of the ability to wake a sleeping thread.
Add a new pipe, one per thread, so that we can write on it to wake a thread
sleeping in a poller, and use it to wake threads supposed to take care of a
task, if they are all sleeping.
As __task_wakeup() is responsible for increasing
rqueue_local[tid]/global_rqueue_size, make __task_unlink_rq responsible for
decreasing it, as process_runnable_tasks() isn't the only one that removes
tasks from runqueues.
We may remove the thread's bit in active_tasks_mask despite tasks for that
thread still being present in the global runqueue. To fix that, introduce
global_tasks_mask, and set the correspnding bits when we add a task to the
runqueue.
We need to decrement requeue_size when we remove a task form rqueue_local,
not when we remove if from the task list, or we'd also decrement it for any
tasklet, that was never in the rqueue in the first place.
Commit 09eeb76 ("BUG/MEDIUM: tasks: Don't forget to increase/decrease
tasks_run_queue.") addressed a count issue in the run queue and uncovered
another issue with the way the tasks are dequeued from the global run
queue. The number of tasks to pick is computed using an integral divide,
which results in up to nbthread-1 tasks never being run. The fix simply
consists in getting rid of the divide and checking the task count in the
loop.
No backport is needed, this is 1.9-specific.
We can't just set t to NULL if it's a tasklet, or we'd have a hard time
accessing to t->process, so just make sure we pass NULL as the first parameter
of t->process if it's a tasklet.
This should be a non-issue at this point, as tasklets aren't used yet.
To make sure we don't inadvertently insert task in the global runqueue,
while only the local runqueue is used without threads, make its definition
and usage conditional on USE_THREAD.
We're taking tasks from the global runqueue based on the number of tasks
the thread already have in its local runqueue, but now that we have a task
list, we also have to take that into account.
When the task list was introduced, we bogusly lost max_processed--, that means
we would execute as much tasks as present in the list, and we would never
set active_tasks_mask, so the thread would go to sleep even if more tasks were
to be executed.
1.9-dev only, no backport is needed.
Introduce tasklets, lightweight tasks. They have no notion of priority,
they are just run as soon as possible, and will probably be used for I/O
later.
For the moment they're used to replace the temporary thread-local list
that was used in the scheduler. The first part of the struct is common
with tasks so that tasks can be cast to tasklets and queued in this list.
Once a task is in the tasklet list, it has its leaf_p set to 0x1 so that
it cannot accidently be confused as not in the queue.
Pure tasklets are identifiable by their nice value of -32768 (which is
normally not possible).
A lot of tasks are run on one thread only, so instead of having them all
in the global runqueue, create a per-thread runqueue which doesn't require
any locking, and add all tasks belonging to only one thread to the
corresponding runqueue.
The global runqueue is still used for non-local tasks, and is visited
by each thread when checking its own runqueue. The nice parameter is
thus used both in the global runqueue and in the local ones. The rare
tasks that are bound to multiple threads will have their nice value
used twice (once for the global queue, once for the thread-local one).
In preparation for thread-specific runqueues, change the task API so that
the callback takes 3 arguments, the task itself, the context, and the state,
those were retrieved from the task before. This will allow these elements to
change atomically in the scheduler while the application uses the copied
value, and even to have NULL tasks later.
While running a task, we may try to delete and free a task that is about to
be run, because it's part of the local tasks list, or because rq_next points
to it.
So flag any task that is in the local tasks list to be deleted, instead of
run, by setting t->process to NULL, and re-make rq_next a global,
thread-local variable, that is modified if we attempt to delete that task.
Many thanks to PiBa-NL for reporting this and analysing the problem.
This should be backported to 1.8.
A number of counters have been added at special places helping better
understanding certain bug reports. These counters are maintained per
thread and are shown using "show activity" on the CLI. The "clear
counters" commands also reset these counters. The output is sent as a
single write(), which currently produces up to about 7 kB of data for
64 threads. If more counters are added, it may be necessary to write
into multiple buffers, or to reset the counters.
To backport to 1.8 to help collect more detailed bug reports.
We really don't want them to share the same cache line as they are
expected to be used in parallel. Adding a 64-byte alignment here shows
a performance increase of about 4.5% on task-intensive workloads with
2 to 4 threads.
Very often when debugging, the current task's pointer isn't easy to
recover (eg: from a core file). Let's keep a copy of it, it will
likely help, especially with threads.
During the migration to the second version of the pools, the new
functions and pool pointers were all called "pool_something2()" and
"pool2_something". Now there's no more pool v1 code and it's a real
pain to still have to deal with this. Let's clean this up now by
removing the "2" everywhere, and by renaming the pool heads
"pool_head_something".
There is a small unprotected window for a task between the wait queue
and the run queue where a task could be woken up and destroyed at the
same time. What typically happens is that a timeout is reached at the
same time an I/O completes and wakes it up, and the I/O terminates the
task, causing a use after free in wake_expired_tasks() possibly causing
a crash and/or memory corruption :
thread 1 thread 2
(wake_expired_tasks) (stream_int_notify)
HA_SPIN_UNLOCK(TASK_WQ_LOCK, &wq_lock);
task_wakeup(task, TASK_WOKEN_IO);
...
process_stream()
stream_free()
task_free()
pool_free(task)
task_wakeup(task, TASK_WOKEN_TIMER);
This case is reasonably easy to reproduce with a config using very short
server timeouts (100ms) and client timeouts (10ms), while injecting on
httpterm requesting medium sized objects (5kB) over SSL. All this is
easier done with more threads than allocated CPUs so that pauses can
happen anywhere and last long enough for process_stream() to kill the
task.
This patch inverts the lock and the wakeup(), but requires some changes
in process_runnable_tasks() to ensure we never try to grab the WQ lock
while having the RQ lock held. This means we have to release the RQ lock
before calling task_queue(), so we can't hold the RQ lock during the
loop and must take and drop it.
It seems that a different approach with the scope-aware trees could be
easier, but it would possibly not cover situations where a task is
allowed to run on multiple threads. The current solution covers it and
doesn't seem to have any measurable performance impact.
tasks_run_queue is the run queue size. It is a global variable. So it is
underoptimized because we may be lead to consider there are active tasks for a
thread while in fact all active tasks are assigned to the other threads. So, in
such cases, the polling loop will be evaluated many more times than necessary.
Instead, we now check if the thread id is set in the bitfield active_tasks_mask.
Another change has been made in process_runnable_tasks. Now, we always limit the
number of tasks processed to 200.
This is specific to threads, no backport is needed.
a bitfield has been added to know if there are runnable tasks for a thread. When
a task is woken up, the bits corresponding to its thread_mask are set. When all
tasks for a thread have been evaluated without any wakeup, the thread is removed
from active ones by unsetting its tid_bit from the bitfield.
This macro should be used to declare variables or struct members depending on
the USE_THREAD compile option. It avoids the encapsulation of such declarations
between #ifdef/#endif. It is used to declare all lock variables.
My recent change in commit ce4e0aa ("MEDIUM: task: change the construction
of the loop in process_runnable_tasks()") was bogus as it used to keep the
rq_next across an unlock/lock sequence, occasionally leading to crashes for
tasks that are eligible to any thread. We must use the lookup call for each
new batch instead. The problem is easily triggered with such a configuration :
global
nbthread 4
listen check
mode http
bind 0.0.0.0:8080
redirect location /
option httpchk GET /
server s1 127.0.0.1:8080 check inter 1
server s2 127.0.0.1:8080 check inter 1
Thanks to Olivier for diagnosing this one. No backport is needed.
The scheduler is complex and uses local queues to amortize the cost of
locks. But all this comes with a cost that is quite observable with
single-thread workloads.
The purpose of this patch is to reimplement the much simpler scheduler
for the case where threads are not used. The code is very small and
simple. It doesn't impact the multi-threaded performance at all, and
provides a nice 10% performance increase in single-thread by reaching
606kreq/s on the tests that showed 550kreq/s before.
process_runnable_tasks() needs to requeue or wake up tasks after
processing them in batches. By only refilling the existing ones, we
avoid revisiting all the queue. The performance gain is measurable
starting with two threads, where the request rate climbs to 657k/s
compared to 644k.
This patch slightly rearranges the loop to pack the locked code a little
bit, and to try to concentrate accesses to the tree together to benefit
more from the cache.
It also fixes how the loop handles the right margin : now that is guaranteed
that the retrieved nodes are filtered to only match the current thread, we
don't need to rewind every 16 entries. Instead we can rewind each time we
reach the right margin again.
With this change, we now achieve the following performance for 10 H2 conns
each containing 100 streams :
1 thread : 550kreq/s
2 thread : 644kreq/s
3 thread : 598kreq/s
This function is sensitive, let's make it shorter by factoring out the
unlock and leave code. This reduced the function's size by a few tens
of bytes and increased the overall performance by about 1%.
Currently the task scheduler suffers from an O(n) lookup when
skipping tasks that are not for the current thread. The reason
is that eb32_lookup_ge() has no information about the current
thread so it always revisits many tasks for other threads before
finding its own tasks.
This is particularly visible with HTTP/2 since the number of
concurrent streams created at once causes long series of tasks
for the same stream in the scheduler. With only 10 connections
and 100 streams each, by running on two threads, the performance
drops from 640kreq/s to 11.2kreq/s! Lookup metrics show that for
only 200000 task lookups, 430 million skips had to be performed,
which means that on average, each lookup leads to 2150 nodes to
be visited.
This commit backports the principle of scope lookups for ebtrees
from the ebtree_v7 development tree. The idea is that each node
contains a mask indicating the union of the scopes for the nodes
below it, which is fed during insertion, and used during lookups.
Then during lookups, branches that do not contain any leaf matching
the requested scope are simply ignored. This perfectly matches a
thread mask, allowing a thread to only extract the tasks it cares
about from the run queue, and to always find them in O(log(n))
instead of O(n). Thus the scheduler uses tid_bit and
task->thread_mask as the ebtree scope here.
Doing this has recovered most of the performance, as can be seen on
the test below with two threads, 10 connections, 100 streams each,
and 1 million requests total :
Before After Gain
test duration : 89.6s 4.73s x19
HTTP requests/s (DEBUG) : 11200 211300 x19
HTTP requests/s (PROD) : 15900 447000 x28
spin_lock time : 85.2s 0.46s /185
time per lookup : 13us 40ns /325
Even when going to 6 threads (on 3 hyperthreaded CPU cores), the
performance stays around 284000 req/s, showing that the contention
is much lower.
A test showed that there's no benefit in using this for the wait queue
though.
It was a leftover from the last cleaning session; this mask applies
to threads and calling it process_mask is a bit confusing. It's the
same in fd, task and applets.
