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SubjectRe: Heads-up: two regressions in v4.11-rc series
On Thu, Apr 20, 2017 at 11:00:42AM +0200, Jesper Dangaard Brouer wrote:
> Hi Linus,
>
> Just wanted to give a heads-up on two regressions in 4.11-rc series.
>
> (1) page allocator optimization revert
>
> Mel Gorman and I have been playing with optimizing the page allocator,
> but Tariq spotted that we caused a regression for (NIC) drivers that
> refill DMA RX rings in softirq context.
>
> The end result was a revert, and this is waiting in AKPMs quilt queue:
> http://ozlabs.org/~akpm/mmots/broken-out/revert-mm-page_alloc-only-use-per-cpu-allocator-for-irq-safe-requests.patch
>
>
> (2) Busy softirq can cause userspace not to be scheduled
>
> I bisected the problem to a499a5a14dbd ("sched/cputime: Increment
> kcpustat directly on irqtime account"). See email thread with
> Subject: Bisected softirq accounting issue in v4.11-rc1~170^2~28
> http://lkml.kernel.org/r/20170328101403.34a82fbf@redhat.com
>
> I don't know the scheduler code well enough to fix this, and will have
> to rely others to figure out this scheduler regression.
>
> To make it clear: I'm only seeing this scheduler regression when a
> remote host is sending many many network packets, towards the kernel
> which keeps NAPI/softirq busy all the time. A possible hint: tool
> "top" only shows this in "si" column, while on v4.10 "top" also blames
> "ksoftirqd/N", plus "ps" reported cputime (0:00) seems wrong for ksoftirqd.

(I'm currently working on reproducing that one.)

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