linux-kernel - Re: [RFC PATCH v3] sched: Fix performance regression introduced by mm

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Date:   Tue, 11 Apr 2023 08:41:25 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Aaron Lu <aaron.lu@...el.com>
Subject: Re: [RFC PATCH v3] sched: Fix performance regression introduced by
 mm_cid

On 2023-04-11 04:53, Peter Zijlstra wrote:
> On Wed, Apr 05, 2023 at 12:26:35PM -0400, Mathieu Desnoyers wrote:
>> +void sched_mm_cid_migrate_from(struct task_struct *t)
>> +{
> 
>> +	/*
>> +	 * If the source cpu cid is set, and matches the last cid of the
>> +	 * migrated task, clear the source cpu cid to keep cid allocation
>> +	 * compact to cover the case where this task is the last task using
>> +	 * this mm on the source cpu. If there happens to be other tasks left
>> +	 * on the source cpu using this mm, the next task using this mm will
>> +	 * reallocate its cid on context switch.
>> +	 *
>> +	 * We cannot keep ownership of concurrency ID without runqueue
>> +	 * lock held when it is not used by a current task, because it
>> +	 * would lead to allocation of more concurrency ids than there
>> +	 * are possible cpus in the system. The last_mm_cid is used as
>> +	 * a hint to conditionally unset the dst cpu cid, keeping
>> +	 * allocated concurrency ids compact.
>> +	 */
>> +	if (cmpxchg(src_pcpu_cid, src_cid, mm_cid_set_lazy_put(src_cid)) != src_cid)
>> +		return;
>> +
> 
> FWIW, I'm thinking that if we write this using try_cmpxchg() it'll be a
> little nicer:
> 
> 	lazy_cid = mm_cid_set_lazy_put(src_cid);
> 	if (!try_cmpxchg(src_pcpu_cid, &src_cid, lazy_cid))
> 		return;
> 

Yes, done.

>> +	/*
>> +	 * The implicit barrier after cmpxchg per-mm/cpu cid before loading
>> +	 * rq->curr->mm matches the scheduler barrier in context_switch()
>> +	 * between store to rq->curr and load of prev and next task's
>> +	 * per-mm/cpu cid.
>> +	 *
>> +	 * The implicit barrier after cmpxchg per-mm/cpu cid before loading
>> +	 * rq->curr->mm_cid_active matches the barrier in
>> +	 * sched_mm_cid_exit_signals(), sched_mm_cid_before_execve(), and
>> +	 * sched_mm_cid_after_execve() between store to t->mm_cid_active and
>> +	 * load of per-mm/cpu cid.
>> +	 */
>> +
>> +	/*
>> +	 * If we observe an active task using the mm on this rq after setting the lazy-put
>> +	 * flag, this task will be responsible for transitioning from lazy-put
>> +	 * flag set to MM_CID_UNSET.
>> +	 */
>> +	rcu_read_lock();
>> +	src_task = rcu_dereference(src_rq->curr);
>> +	if (src_task->mm_cid_active && src_task->mm == mm) {
>> +		rcu_read_unlock();
>> +		/*
>> +		 * We observed an active task for this mm, clearing the destination
>> +		 * cpu mm_cid is not relevant for compactness.
>> +		 */
>> +		t->last_mm_cid = -1;
>> +		return;
>> +	}
>> +	rcu_read_unlock();
>> +
>> +	/*
>> +	 * The src_cid is unused, so it can be unset.
>> +	 */
>> +	if (cmpxchg(src_pcpu_cid, mm_cid_set_lazy_put(src_cid), MM_CID_UNSET) != mm_cid_set_lazy_put(src_cid))
>> +		return;
> 
> 	if (!try_cmpxchg(src_pcpu_cid, &lazy_cid, MM_CID_UNSET))
> 		return;

done,

Thanks!

Mathieu

> 
>> +	__mm_cid_put(mm, src_cid);
>> +}

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com