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SubjectRe: [PATCH] softirq: let ksoftirqd do its job
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On 31.08.2016 19:42, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@google.com>
>
> A while back, Paolo and Hannes sent an RFC patch adding threaded-able
> napi poll loop support : (https://patchwork.ozlabs.org/patch/620657/)
>
> The problem seems to be that softirqs are very aggressive and are often
> handled by the current process, even if we are under stress and that
> ksoftirqd was scheduled, so that innocent threads would have more chance
> to make progress.
>
> This patch makes sure that if ksoftirq is running, we let it
> perform the softirq work.
>
> Jonathan Corbet summarized the issue in https://lwn.net/Articles/687617/
>
> Tested:
>
> - NIC receiving traffic handled by CPU 0
> - UDP receiver running on CPU 0, using a single UDP socket.
> - Incoming flood of UDP packets targeting the UDP socket.
>
> Before the patch, the UDP receiver could almost never get cpu cycles and
> could only receive ~2,000 packets per second.
>
> After the patch, cpu cycles are split 50/50 between user application and
> ksoftirqd/0, and we can effectively read ~900,000 packets per second,
> a huge improvement in DOS situation. (Note that more packets are now
> dropped by the NIC itself, since the BH handlers get less cpu cycles to
> drain RX ring buffer)
>
> Since the load runs in well identified threads context, an admin can
> more easily tune process scheduling parameters if needed.
>
> Reported-by: Paolo Abeni <pabeni@redhat.com>
> Reported-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Cc: David Miller <davem@davemloft.net
> Cc: Jesper Dangaard Brouer <jbrouer@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rik van Riel <riel@redhat.com>

Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>

Thanks,
Hannes


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