netdev - Heads-up: two regressions in v4.11-rc series

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Date:   Thu, 20 Apr 2017 11:00:42 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Frederic Weisbecker <fweisbec@...il.com>
Cc:     brouer@...hat.com, Mel Gorman <mgorman@...hsingularity.net>,
        Tariq Toukan <tariqt@...lanox.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        peterz@...radead.org
Subject: Heads-up: two regressions in v4.11-rc series

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.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer