Re: [PATCH -V2] NUMA balancing: reduce TLB flush via delaying mapping on hint page fault

From: "Huang, Ying" <ying.huang@intel.com>
To: Mel Gorman <mgorman@suse.de>, Andrew Morton <akpm@linux-foundation.org>
Cc: <linux-mm@kvack.org>,  <linux-kernel@vger.kernel.org>,
	 Peter Zijlstra <peterz@infradead.org>,
	 "Peter Xu" <peterx@redhat.com>,
	 Johannes Weiner <hannes@cmpxchg.org>,
	 "Vlastimil Babka" <vbabka@suse.cz>,
	 Matthew Wilcox <willy@infradead.org>,
	 Will Deacon <will@kernel.org>,
	 Michel Lespinasse <walken@google.com>,
	 Arjun Roy <arjunroy@google.com>,
	 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH -V2] NUMA balancing: reduce TLB flush via delaying mapping on hint page fault
Date: Thu, 08 Apr 2021 21:46:11 +0800	[thread overview]
Message-ID: <87lf9syjmk.fsf@yhuang6-desk1.ccr.corp.intel.com> (raw)
In-Reply-To: <20210407082728.GA15768@suse.de> (Mel Gorman's message of "Wed, 7 Apr 2021 09:27:28 +0100")

Mel Gorman <mgorman@suse.de> writes:

> On Fri, Apr 02, 2021 at 04:27:17PM +0800, Huang Ying wrote:
>> With NUMA balancing, in hint page fault handler, the faulting page
>> will be migrated to the accessing node if necessary.  During the
>> migration, TLB will be shot down on all CPUs that the process has run
>> on recently.  Because in the hint page fault handler, the PTE will be
>> made accessible before the migration is tried.  The overhead of TLB
>> shooting down can be high, so it's better to be avoided if possible.
>> In fact, if we delay mapping the page until migration, that can be
>> avoided.  This is what this patch doing.
>> 
>> <SNIP>
>>
>
> Thanks, I think this is ok for Andrew to pick up to see if anything
> bisects to this commit but it's a low risk.
>
> Reviewed-by: Mel Gorman <mgorman@suse.de>
>
> More notes;
>
> This is not a universal win given that not all workloads exhibit the
> pattern where accesses occur in parallel threads between when a page
> is marked accessible and when it is migrated. The impact of the patch
> appears to be neutral for those workloads. For workloads that do exhibit
> the pattern, there is a small gain with a reduction in interrupts as
> advertised unlike v1 of the patch. Further tests are running to confirm
> the reduction is in TLB shootdown interrupts but I'm reasonably confident
> that will be the case. Gains are typically small and the load described in
> the changelog appears to be a best case scenario but a 1-5% gain in some
> other workloads is still an improvement. There is still the possibility
> that some workloads will unnecessarily stall as a result of the patch
> for slightly longer periods of time but that is a relatively low risk
> and will be difficult to detect. If I'm wrong, a bisection will find it.

Hi, Mel,

Thanks!

Hi, Andrew,

I found that V2 cannot apply on top of latest mmotm, so I send V3 as
follows.  In case you need it.

https://lore.kernel.org/lkml/20210408132236.1175607-1-ying.huang@intel.com/

Best Regards,
Huang, Ying