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From: Chris Mason <clm@fb.com>
To: Peter Zijlstra <peterz@infradead.org>, <mingo@kernel.org>,
	<tglx@linutronix.de>, <riel@redhat.com>, <hpa@zytor.com>,
	<efault@gmx.de>, <linux-kernel@vger.kernel.org>,
	<torvalds@linux-foundation.org>, <lvenanci@redhat.com>
Cc: <xiaolong.ye@intel.com>, <kitsunyan@inbox.ru>,
	<matt@codeblueprint.co.uk>
Subject: Re: hackbench vs select_idle_sibling; was: [tip:sched/core] sched/fair, cpumask: Export for_each_cpu_wrap()
Date: Wed, 17 May 2017 10:49:46 -0400
Message-ID: <8e44a5e1-35b0-7895-f313-c9a765e20b11@fb.com> (raw)
In-Reply-To: <20170517105350.hk5m4h4jb6dfr65a@hirez.programming.kicks-ass.net>

On 05/17/2017 06:53 AM, Peter Zijlstra wrote:
> On Mon, May 15, 2017 at 02:03:11AM -0700, tip-bot for Peter Zijlstra wrote:
>> sched/fair, cpumask: Export for_each_cpu_wrap()
>
>> -static int cpumask_next_wrap(int n, const struct cpumask *mask, int start, int *wrapped)
>> -{
>
>> -	next = find_next_bit(cpumask_bits(mask), nr_cpumask_bits, n+1);
>
>> -}
>
> OK, so this patch fixed an actual bug in the for_each_cpu_wrap()
> implementation. The above 'n+1' should be 'n', and the effect is that
> it'll skip over CPUs, potentially resulting in an iteration that only
> sees every other CPU (for a fully contiguous mask).
>
> This in turn causes hackbench to further suffer from the regression
> introduced by commit:
>
>   4c77b18cf8b7 ("sched/fair: Make select_idle_cpu() more aggressive")
>
> So its well past time to fix this.
>
> Where the old scheme was a cliff-edge throttle on idle scanning, this
> introduces a more gradual approach. Instead of stopping to scan
> entirely, we limit how many CPUs we scan.
>
> Initial benchmarks show that it mostly recovers hackbench while not
> hurting anything else, except Mason's schbench, but not as bad as the
> old thing.
>
> It also appears to recover the tbench high-end, which also suffered like
> hackbench.
>
> I'm also hoping it will fix/preserve kitsunyan's interactivity issue.
>
> Please test..

We'll get some tests going here too.

-chris

  parent reply index

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-04-13 13:56 [RFC 0/3] sched/topology: fix sched groups on NUMA machines with mesh topology Lauro Ramos Venancio
2017-04-13 13:56 ` [RFC 1/3] sched/topology: Refactor function build_overlap_sched_groups() Lauro Ramos Venancio
2017-04-13 14:50   ` Rik van Riel
2017-05-15  9:02   ` [tip:sched/core] " tip-bot for Lauro Ramos Venancio
2017-04-13 13:56 ` [RFC 2/3] sched/topology: fix sched groups on NUMA machines with mesh topology Lauro Ramos Venancio
2017-04-13 15:16   ` Rik van Riel
2017-04-13 15:48   ` Peter Zijlstra
2017-04-13 20:21     ` Lauro Venancio
2017-04-13 21:06       ` Lauro Venancio
2017-04-13 23:38         ` Rik van Riel
2017-04-14 10:48           ` Peter Zijlstra
2017-04-14 11:38   ` Peter Zijlstra
2017-04-14 12:20     ` Peter Zijlstra
2017-05-15  9:03       ` [tip:sched/core] sched/fair, cpumask: Export for_each_cpu_wrap() tip-bot for Peter Zijlstra
2017-05-17 10:53         ` hackbench vs select_idle_sibling; was: " Peter Zijlstra
2017-05-17 12:46           ` Matt Fleming
2017-05-17 14:49           ` Chris Mason [this message]
2017-05-19 15:00           ` Matt Fleming
2017-06-05 13:00             ` Matt Fleming
2017-06-06  9:21               ` Peter Zijlstra
2017-06-09 17:52                 ` Chris Mason
2017-06-08  9:22           ` [tip:sched/core] sched/core: Implement new approach to scale select_idle_cpu() tip-bot for Peter Zijlstra
2017-04-14 16:58     ` [RFC 2/3] sched/topology: fix sched groups on NUMA machines with mesh topology Peter Zijlstra
2017-04-17 14:40       ` Lauro Venancio
2017-04-13 13:56 ` [RFC 3/3] sched/topology: Different sched groups must not have the same balance cpu Lauro Ramos Venancio
2017-04-13 15:27   ` Rik van Riel
2017-04-14 16:49   ` Peter Zijlstra
2017-04-17 15:34     ` Lauro Venancio
2017-04-18 12:32       ` Peter Zijlstra

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