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Date: Tue, 13 Feb 2018 15:43:26 +0100 From: Peter Zijlstra <peterz@...radead.org> To: Mel Gorman <mgorman@...hsingularity.net> Cc: Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>, Matt Fleming <matt@...eblueprint.co.uk>, Giovanni Gherdovich <ggherdovich@...e.cz>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 6/6] sched/numa: Delay retrying placement for automatic NUMA balance after wake_affine On Tue, Feb 13, 2018 at 02:18:12PM +0000, Mel Gorman wrote: > On Tue, Feb 13, 2018 at 03:01:37PM +0100, Peter Zijlstra wrote: > > On Tue, Feb 13, 2018 at 01:37:30PM +0000, Mel Gorman wrote: > > > +static void > > > +update_wa_numa_placement(struct task_struct *p, int prev_cpu, int target) > > > +{ > > > + unsigned long interval; > > > + > > > + if (!static_branch_likely(&sched_numa_balancing)) > > > + return; > > > + > > > + /* If balancing has no preference then continue gathering data */ > > > + if (p->numa_preferred_nid == -1) > > > + return; > > > + > > > + /* > > > + * If the wakeup is not affecting locality then it is neutral from > > > + * the perspective of NUMA balacing so continue gathering data. > > > + */ > > > + if (cpus_share_cache(prev_cpu, target)) > > > + return; > > > > Dang, I wanted to mention this before, but it slipped my mind. The > > comment and code don't match. > > > > Did you want to write: > > > > if (cpu_to_node(prev_cpu) == cpu_to_node(target)) > > return; > > > > Well, it was deliberate. While it's possible to be on the same memory > node and not sharing cache, the scheduler typically is more concerned with > the LLC than NUMA per-se. If they share LLC, then I also assume that they > share memory locality. True, but the remaining code only has effect for numa balance, which is concerned with nodes. So I don't see the point of using something potentially smaller. Suppose someone did hardware where a node has 2 cache clusters, then we'd still set a wake_affine back-off for numa-balance, even though it remains on the same node. How would that be useful?
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