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Message-ID: <fb2b7e96-23b8-449e-93fe-88ee3ea167d1@efficios.com>
Date: Thu, 5 Sep 2024 09:36:49 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Yury Norov <yury.norov@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
linux-kernel@...r.kernel.org, Valentin Schneider <vschneid@...hat.com>,
Mel Gorman <mgorman@...e.de>, Steven Rostedt <rostedt@...dmis.org>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>, Ben Segall
<bsegall@...gle.com>, Rasmus Villemoes <linux@...musvillemoes.dk>,
Dmitry Vyukov <dvyukov@...gle.com>, Marco Elver <elver@...gle.com>
Subject: Re: [RFC PATCH 2/2] sched: Improve cache locality of RSEQ concurrency
IDs for intermittent workloads
On 2024-09-04 14:28, Mathieu Desnoyers wrote:
> On 2024-09-04 11:50, Mathieu Desnoyers wrote:
>> On 2024-09-04 11:24, Yury Norov wrote:
> [...]
>>>
>>> This all doesn't look like a hot path. And anyways, speculating around
>>> performance without numbers on hands sounds cheap.
>>
>> This is done whenever userspace invokes sched_setaffinity, or changes
>> its cgroup cpuset. It may not be the most important fast-path in the
>> world, but I expect some workloads to issue sched_setaffinity whenever
>> they create a thread, so it's not a purely slow-path either.
>>
>>> In my experience, iterators with a very lightweight payload are ~100
>>> times slower comparing to dedicated bitmap ops. Check this for example:
>>> 3cea8d4753277.
>>>
>>> If you're really cared about performance here, I'd suggest you to
>>> compare your iterators approach with something like this:
>>>
>>> cpumask_or(mm_allowed, mm_allowed, cpumask);
>>> atomic_set(&mm->nr_cpus_allowed, cpumask_weight(mm_allowed);
>
> Here are the benchmark results. Each test use two entirely filled
> bitmaps as input to mimic the common scenario for cpus allowed
> being updated with a subset of the original process CPUs allowed,
> and also the common case where the initial cpumask is filled.
>
> #define BITMAP_LEN (4096UL * 8 * 10)
> (len = BITMAP_LEN)
>
> * Approach 1:
>
> int nr_set = 0;
> for_each_andnot_bit(bit, bitmap, bitmap2, len)
> nr_set += !test_and_set_bit(bit, bitmap2);
> if (nr_set)
> atomic_add(nr_set, &total);
>
> Time: 4680 ns
>
> * Approach 2:
>
> int nr_set = 0;
> for_each_set_bit(bit, bitmap, len)
> nr_set += !test_and_set_bit(bit, bitmap2);
> if (nr_set)
> atomic_add(nr_set, &total);
>
> Time: 1791537 ns
>
> * Approach 3:
>
> mutex_lock(&lock);
> bitmap_or(bitmap2, bitmap, bitmap2, len);
> atomic_set(&total, bitmap_weight(bitmap2, len));
> mutex_unlock(&lock);
>
> Time: 79591 ns
The benchmark result is wrong for approach 3, as it was taken with
CONFIG_PROVE_LOCKING=y and lockdep.
Corrected result:
Time: 4500 ns.
So let's go with your approach. I'm wondering whether I should
re-use an existing mutex/spinlock from mm_struct or add a new one.
Thanks,
Mathieu
>
> The test hardware is a AMD EPYC 9654 96-Core Processor.
>
> Thanks,
>
> Mathieu
>
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
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