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Date: Thu, 13 Apr 2023 11:27:47 -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>,
Olivier Dion <odion@...icios.com>, michael.christie@...cle.com
Subject: Re: [RFC PATCH v5] sched: Fix performance regression introduced by
mm_cid
On 2023-04-13 11:10, Peter Zijlstra wrote:
> On Thu, Apr 13, 2023 at 10:02:48AM -0400, Mathieu Desnoyers wrote:
>
>> +static inline void __mm_cid_put(struct mm_struct *mm, int cid)
>> +{
>> + if (cid < 0)
>> + return;
>> + cpumask_clear_cpu(cid, mm_cidmask(mm));
>> +}
>> +
>> +/*
>> + * The per-mm/cpu cid can have the MM_CID_LAZY_PUT flag set or transition to the
>> + * MM_CID_UNSET state without holding the rq lock, but the rq lock needs to be
>> + * held to transition to other states.
>> + */
>> +static inline void mm_cid_put_lazy(struct task_struct *t)
>> +{
>> + struct mm_struct *mm = t->mm;
>> + int *pcpu_cid, cid;
>> +
>> + lockdep_assert_rq_held(this_rq());
>> + pcpu_cid = this_cpu_ptr(mm->pcpu_cid);
>> + cid = READ_ONCE(*pcpu_cid);
>> + if (!mm_cid_is_lazy_put(cid))
>> + return;
>> + if (!try_cmpxchg(pcpu_cid, &cid, MM_CID_UNSET))
>> + return;
>> + __mm_cid_put(mm, mm_cid_clear_lazy_put(cid));
>> +}
>> +
>> +static inline void mm_cid_put(struct task_struct *t)
>> +{
>> + struct mm_struct *mm = t->mm;
>> + int *pcpu_cid, cid, res;
>> +
>> + lockdep_assert_rq_held(this_rq());
>> + WARN_ON_ONCE(t->mm_cid < 0);
>> + pcpu_cid = this_cpu_ptr(mm->pcpu_cid);
>> + cid = READ_ONCE(*pcpu_cid);
>> + for (;;) {
>> + if (mm_cid_is_unset(cid))
>> + return;
>> + WARN_ON_ONCE(mm_cid_clear_lazy_put(cid) != t->mm_cid);
>> + /*
>> + * Attempt transition from valid or lazy-put to unset.
>> + */
>> + res = cmpxchg(pcpu_cid, cid, MM_CID_UNSET);
>> + if (res == cid)
>> + break;
>> + cid = res;
>
> if (try_cmpxchg(pcpu_cid, &cid, MM_CID_UNSET))
> break;
I left this as a cmpxchg on purpose: it acts as a read for the next
loop. Or is it better to move the "cid = READ_ONCE(*pcpu_cid);" at the
beginning of the loop and use try_cmpxchg ?
>
>> + }
>
>
>
>> + __mm_cid_put(mm, mm_cid_clear_lazy_put(cid));
>> +}
>> +
>> +static inline int __mm_cid_try_get(struct mm_struct *mm)
>> {
>> struct cpumask *cpumask;
>> int cid;
>>
>> cpumask = mm_cidmask(mm);
>> + /*
>> + * Retry finding first zero bit if the mask is temporarily
>> + * filled. This only happens during concurrent migrate-from
>> + * which owns a cid without holding a rq lock.
>> + */
>> + do {
>> + cid = cpumask_first_zero(cpumask);
>> + } while (cid >= nr_cpu_ids);
>
> Urgh..
> for (;;) {
> cid = cpumask_first_zero(cpumask);
> if (cid < nr_cpu_ids)
> break;
> cpu_relax();
> }
OK
>
>
>> + if (!cpumask_test_and_set_cpu(cid, cpumask))
>> + return cid;
>> + /*
>> + * If initial find-then-test-and-set fails due to concurrent updates,
>> + * attempt a brute-force test-and-set from 0 to nr_cpu_ids-1 to
>> + * eliminate the race between find and test-and-set. This does not
>> + * strictly provide forward-progress guarantees, because we could
>> + * theoretically be racing against concurrent updates of the mask, but
>> + * it does have much better odds to succeed in contended cases than
>> + * the find-then-test-and-set approach.
>> + */
>> + for (cid = 0; cid < nr_cpu_ids; cid++) {
>> + if (!cpumask_test_and_set_cpu(cid, cpumask))
>> + return cid;
>> + }
>
> Yikes, we have IRQs disabled and hold rq->lock, this is a very expensive
> proposition. Also, afaict all *should* just work without this on.
Yes, I'll remove this.
>
>> + return -1;
>> }
>>
>> +static inline int __mm_cid_get(struct mm_struct *mm)
>> {
>> + int cid;
>> +
>> + if (!READ_ONCE(use_cid_lock)) {
>
> This all hurts my brain, but shouldn't you be able to replace
> use_cid_lock with raw_spin_is_locked(&cid_lock) ?
No, because we only want the thread which observed the find/test-and-set
race to set the use_cid_lock bit. We don't want all other threads which
end up using the lock because they see use_cid_lock set to also
themselves cause other threads to use the lock. This could cause a
scenario where we always keep using the lock and never revert back to
lock-free in a very contended case.
>
>> + cid = __mm_cid_try_get(mm);
>> + if (cid >= 0)
>> + goto end;
>> + raw_spin_lock(&cid_lock);
>> + } else {
>> + raw_spin_lock(&cid_lock);
>> + cid = __mm_cid_try_get(mm);
>> + if (cid >= 0)
>> + goto unlock;
>> + }
>> +
>> + /*
>> + * cid concurrently allocated. Retry while forcing following
>> + * allocations to use the cid_lock to ensure forward progress.
>> + */
>> + WRITE_ONCE(use_cid_lock, 1);
>> + /*
>> + * Set use_cid_lock before allocation. Only care about program order
>> + * because this is for forward progress.
>> + */
>> + barrier();
>> + /*
>> + * Retry until it succeeds. It is guaranteed to eventually
>> + * succeed once all newcoming allocations observe the
>> + * use_cid_lock flag set.
>> + */
>> + do {
>> + cid = __mm_cid_try_get(mm);
>> + } while (cid < 0);
>> + /*
>> + * Allocate before clearing use_cid_lock. Only care about
>> + * program order because this is for forward progress.
>> + */
>> + barrier();
>> + WRITE_ONCE(use_cid_lock, 0);
>> +unlock:
>> + raw_spin_unlock(&cid_lock);
>> +end:
>> + return cid;
>> }
>
>
> Anyway, I ran the thing and __schedule()'s dropped from around 11% to 5%
That's a good start!
>
> 4.98% 0.04% swapper [kernel.vmlinux] [k] schedule_idle
> |
> --4.95%--schedule_idle
> |
> --4.94%--__schedule
> |
> |--2.36%--mm_cid_get
> |
> |--0.72%--finish_task_switch.isra.0
> | |
> | --0.61%--asm_sysvec_call_function_single
> | |
> | --0.51%--sysvec_call_function_single
> |
> --0.59%--switch_mm_irqs_off
>
> So improved, but not as good as my glorious hack ;-)
Yeah, we still need to touch the bitmap and suffer from cache line bouncing.
>
> Also, I see that even migrate_from is visible
>
> 5.26% 0.74% sched-messaging [kernel.vmlinux] [k] try_to_wake_up
> |
> |--4.52%--try_to_wake_up
> | |
> | |--1.49%--set_task_cpu
> | | |
> | | --1.18%--sched_mm_cid_migrate_from
> | |
> | |--1.24%--select_task_rq_fair
> | |
> | |--0.73%--ttwu_do_activate
> | | |
> | | --0.57%--enqueue_task_fair
> | |
> | --0.50%--ttwu_queue_wakelist
> |
> --0.74%--start_thread
One tradeoff we can consider is that we could afford to delay compaction
of cids to much later. One way to achieve this would be to update a
timestamp along with each per-mm/cpu cid value keeping track of its last
use on context switch. Then we could implement a worker thread which
would periodically iterate over all mm and figure out if it needs to
compact things because per-mm/cpu cids were unused for too long.
Thoughts ?
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
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