Re: [PATCH v3] sched/fair: bring back select_idle_smt, but differently

[Mel Gorman <mgorman@techsingularity.net>]

On Wed, Apr 07, 2021 at 09:17:18AM +0200, Peter Zijlstra wrote:
> Subject: sched/fair: Bring back select_idle_smt(), but differently
> From: Rik van Riel <riel@surriel.com>
> Date: Fri, 26 Mar 2021 15:19:32 -0400
> 
> From: Rik van Riel <riel@surriel.com>
> 
> Mel Gorman did some nice work in 9fe1f127b913 ("sched/fair: Merge
> select_idle_core/cpu()"), resulting in the kernel being more efficient
> at finding an idle CPU, and in tasks spending less time waiting to be
> run, both according to the schedstats run_delay numbers, and according
> to measured application latencies. Yay.
> 
> The flip side of this is that we see more task migrations (about 30%
> more), higher cache misses, higher memory bandwidth utilization, and
> higher CPU use, for the same number of requests/second.
> 
> This is most pronounced on a memcache type workload, which saw a
> consistent 1-3% increase in total CPU use on the system, due to those
> increased task migrations leading to higher L2 cache miss numbers, and
> higher memory utilization. The exclusive L3 cache on Skylake does us
> no favors there.
> 
> On our web serving workload, that effect is usually negligible.
> 
> It appears that the increased number of CPU migrations is generally a
> good thing, since it leads to lower cpu_delay numbers, reflecting the
> fact that tasks get to run faster. However, the reduced locality and
> the corresponding increase in L2 cache misses hurts a little.
> 
> The patch below appears to fix the regression, while keeping the
> benefit of the lower cpu_delay numbers, by reintroducing
> select_idle_smt with a twist: when a socket has no idle cores, check
> to see if the sibling of "prev" is idle, before searching all the
> other CPUs.
> 
> This fixes both the occasional 9% regression on the web serving
> workload, and the continuous 2% CPU use regression on the memcache
> type workload.
> 
> With Mel's patches and this patch together, task migrations are still
> high, but L2 cache misses, memory bandwidth, and CPU time used are
> back down to what they were before. The p95 and p99 response times for
> the memcache type application improve by about 10% over what they were
> before Mel's patches got merged.
> 
> Signed-off-by: Rik van Riel <riel@surriel.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Link: https://lkml.kernel.org/r/20210326151932.2c187840@imladris.surriel.com

I think this is still ok and should not invalidate the previous tests on
v3. While test_idle_cores() was checked on target, as long as target/prev
share cache, the test should be equivalent other than there is a minor
race so

Reviewed-by: Mel Gorman <mgorman@techsingularity.net>

One minor question below though which previously confused me.

> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6112,6 +6112,27 @@ static int select_idle_core(struct task_
>  	return -1;
>  }
>  
> +/*
> + * Scan the local SMT mask for idle CPUs.
> + */
> +static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target)
> +{
> +	int cpu;
> +
> +	if (!static_branch_likely(&sched_smt_present))
> +		return -1;
> +
> +	for_each_cpu(cpu, cpu_smt_mask(target)) {
> +		if (!cpumask_test_cpu(cpu, p->cpus_ptr) ||
> +		    !cpumask_test_cpu(cpu, sched_domain_span(sd)))
> +			continue;

While I know that !cpumask_test_cpu(cpu, sched_domain_span(sd)) was
done previously, I found it hard to believe that the test matters. If
target/prev share a the LLC domain, why would the SMT siblings *not*
share a LLC?

-- 
Mel Gorman
SUSE Labs