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Message-ID: <87v9efp7cs.fsf@notabene.neil.brown.name>
Date: Mon, 09 Nov 2020 13:54:59 +1100
From: NeilBrown <neilb@...e.de>
To: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>
cc: linux-kernel@...r.kernel.org
Subject: [PATCH rfc] workqueue: honour cond_resched() more effectively.
If a worker task for a normal (bound, non-CPU-intensive) calls
cond_resched(), this allows other non-workqueue processes to run, but
does *not* allow other workqueue workers to run. This is because
workqueue will only attempt to run one task at a time on each CPU,
unless the current task actually sleeps.
This is already a problem for should_reclaim_retry() in mm/page_alloc.c,
which inserts a small sleep just to convince workqueue to allow other
workers to run.
It can also be a problem for NFS when closing a very large file (e.g.
100 million pages in memory). NFS can call the final iput() from a
workqueue, which can then take long enough to trigger a workqueue-lockup
warning, and long enough for performance problems to be observed.
I added a WARN_ON_ONCE() in cond_resched() to report when it is run from
a workqueue, and got about 20 hits during boot, many of system_wq (aka
"events") which suggests there is a real chance that worker are being
delayed unnecessarily be tasks which are written to politely relinquish
the CPU.
I think that once a worker calls cond_resched(), it should be treated as
though it was run from a WQ_CPU_INTENSIVE queue, because only cpu-intensive
tasks need to call cond_resched(). This would allow other workers to be
scheduled.
The following patch achieves this I believe.
Signed-off-by: NeilBrown <neilb@...e.de>
---
include/linux/sched.h | 7 ++++++-
include/linux/workqueue.h | 2 ++
kernel/sched/core.c | 4 ++++
kernel/workqueue.c | 16 +++++++++++++++-
mm/page_alloc.c | 12 +-----------
5 files changed, 28 insertions(+), 13 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4418f5cb8324..728870965df1 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1784,7 +1784,12 @@ static inline int test_tsk_need_resched(struct task_struct *tsk)
#ifndef CONFIG_PREEMPTION
extern int _cond_resched(void);
#else
-static inline int _cond_resched(void) { return 0; }
+static inline int _cond_resched(void)
+{
+ if (current->flags & PF_WQ_WORKER)
+ workqueue_cond_resched();
+ return 0;
+}
#endif
#define cond_resched() ({ \
diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index 8b505d22fc0e..7bcc5717d80f 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -626,6 +626,8 @@ static inline bool schedule_delayed_work(struct delayed_work *dwork,
return queue_delayed_work(system_wq, dwork, delay);
}
+void workqueue_cond_resched(void);
+
#ifndef CONFIG_SMP
static inline long work_on_cpu(int cpu, long (*fn)(void *), void *arg)
{
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9a2fbf98fd6f..5b2e38567a0c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5620,6 +5620,8 @@ SYSCALL_DEFINE0(sched_yield)
#ifndef CONFIG_PREEMPTION
int __sched _cond_resched(void)
{
+ if (current->flags & PF_WQ_WORKER)
+ workqueue_cond_resched();
if (should_resched(0)) {
preempt_schedule_common();
return 1;
@@ -5643,6 +5645,8 @@ int __cond_resched_lock(spinlock_t *lock)
int resched = should_resched(PREEMPT_LOCK_OFFSET);
int ret = 0;
+ if (current->flags & PF_WQ_WORKER)
+ workqueue_cond_resched();
lockdep_assert_held(lock);
if (spin_needbreak(lock) || resched) {
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 891ccad5f271..fd2e9557b1a6 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -2296,7 +2296,7 @@ __acquires(&pool->lock)
spin_lock_irq(&pool->lock);
/* clear cpu intensive status */
- if (unlikely(cpu_intensive))
+ if (unlikely(worker->flags & WORKER_CPU_INTENSIVE))
worker_clr_flags(worker, WORKER_CPU_INTENSIVE);
/* tag the worker for identification in schedule() */
@@ -5330,6 +5330,20 @@ int workqueue_set_unbound_cpumask(cpumask_var_t cpumask)
return ret;
}
+void workqueue_cond_resched(void)
+{
+ struct worker *worker = current_wq_worker();
+
+ if (worker && !(worker->flags & WORKER_CPU_INTENSIVE)) {
+ struct worker_pool *pool = worker->pool;
+
+ worker_set_flags(worker, WORKER_CPU_INTENSIVE);
+ if (need_more_worker(pool))
+ wake_up_worker(pool);
+ }
+}
+EXPORT_SYMBOL(workqueue_cond_resched);
+
#ifdef CONFIG_SYSFS
/*
* Workqueues with WQ_SYSFS flag set is visible to userland via
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 13cc653122b7..0d5720ecbfe7 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4420,17 +4420,7 @@ should_reclaim_retry(gfp_t gfp_mask, unsigned order,
}
out:
- /*
- * Memory allocation/reclaim might be called from a WQ context and the
- * current implementation of the WQ concurrency control doesn't
- * recognize that a particular WQ is congested if the worker thread is
- * looping without ever sleeping. Therefore we have to do a short sleep
- * here rather than calling cond_resched().
- */
- if (current->flags & PF_WQ_WORKER)
- schedule_timeout_uninterruptible(1);
- else
- cond_resched();
+ cond_resched();
return ret;
}
--
2.29.2
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