Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 13 Apr 2023 17:20:23 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Aaron Lu <aaron.lu@intel.com>, linux-kernel@vger.kernel.org,
        Olivier Dion <odion@efficios.com>, michael.christie@oracle.com
Subject: Re: [RFC PATCH v4] sched: Fix performance regression introduced by
 mm_cid
Message-ID: <20230413152023.GO4253@hirez.programming.kicks-ass.net>
References: <20230410150150.2179062-1-mathieu.desnoyers@efficios.com>
 <20230411045225.GA3509@ziqianlu-desk2>
 <20230411131221.GA7356@ziqianlu-desk2>
 <20230412091043.GC4253@hirez.programming.kicks-ass.net>
 <20230412114240.GA155547@ziqianlu-desk2>
 <20230412142616.GI628377@hirez.programming.kicks-ass.net>
 <20230412143934.GB162902@ziqianlu-desk2>
 <20230413111047.GB83892@hirez.programming.kicks-ass.net>
 <6b8e63ab-e81e-470c-e03f-f3860c83bdb1@efficios.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <6b8e63ab-e81e-470c-e03f-f3860c83bdb1@efficios.com>
Precedence: bulk

On Thu, Apr 13, 2023 at 09:56:38AM -0400, Mathieu Desnoyers wrote:

> > Mathieu, WDYT? -- other than that the patch is an obvious hack :-)
> 
> I hate it with passion :-)
> 
> It is quite specific to your workload/configuration.
> 
> If we take for instance a process with a large mm_users count which is
> eventually affined to a subset of the cpus with cpusets or
> sched_setaffinity, your patch will prevent compaction of the concurrency ids
> when it really should not.

I don't think it will, it will only kick in once the higest cid is
handed out (I should've used num_online_cpus() instead of nr_cpu_ids),
and with affinity at play that should never happen.

Now, the more fancy scheme with:

	min(t->nr_cpus_allowed, atomic_read(&t->mm->mm_users))

that does get to be more complex; and I've yet to find a working version
that doesn't also need a for_each_cpu() loop on for reclaim :/

Anyway, I think the hack as presented is safe, but a hack none-the-less.