linux-kernel - Re: [PATCH] mm, page_alloc: re-enable softirq use of per-cpu page allocator

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Date:   Mon, 10 Apr 2017 14:26:16 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     brouer@...hat.com, willy@...radead.org, peterz@...radead.org,
        pagupta@...hat.com, ttoukan.linux@...il.com, tariqt@...lanox.com,
        netdev@...r.kernel.org, saeedm@...lanox.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm, page_alloc: re-enable softirq use of per-cpu page
 allocator

On Mon, 10 Apr 2017 16:08:21 +0100 Mel Gorman <mgorman@...hsingularity.net> wrote:

> IRQ context were excluded from using the Per-Cpu-Pages (PCP) lists caching
> of order-0 pages in commit 374ad05ab64d ("mm, page_alloc: only use per-cpu
> allocator for irq-safe requests").
> 
> This unfortunately also included excluded SoftIRQ.  This hurt the performance
> for the use-case of refilling DMA RX rings in softirq context.

Out of curiosity: by how much did it "hurt"?

<ruffles through the archives>

Tariq found:

: I disabled the page-cache (recycle) mechanism to stress the page
: allocator, and see a drastic degradation in BW, from 47.5 G in v4.10 to
: 31.4 G in v4.11-rc1 (34% drop).

then with this patch he found

: It looks very good!  I get line-rate (94Gbits/sec) with 8 streams, in
: comparison to less than 55Gbits/sec before.

Can I take this to mean that the page allocator's per-cpu-pages feature
ended up doubling the performance of this driver?  Better than the
driver's private page recycling?  I'd like to believe that, but am
having trouble doing so ;)

> This patch re-allow softirq context, which should be safe by disabling
> BH/softirq, while accessing the list.  PCP-lists access from both hard-IRQ
> and NMI context must not be allowed.  Peter Zijlstra says in_nmi() code
> never access the page allocator, thus it should be sufficient to only test
> for !in_irq().
> 
> One concern with this change is adding a BH (enable) scheduling point at
> both PCP alloc and free. If further concerns are highlighted by this patch,
> the result wiill be to revert 374ad05ab64d and try again at a later date
> to offset the irq enable/disable overhead.