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