netdev - Re: [PATCH net 2/2] sfc: limit ARFS workitems in flight per channel

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Date:   Thu, 12 Apr 2018 11:11:32 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     ecree@...arflare.com
Cc:     linux-net-drivers@...arflare.com, netdev@...r.kernel.org
Subject: Re: [PATCH net 2/2] sfc: limit ARFS workitems in flight per channel

From: Edward Cree <ecree@...arflare.com>
Date: Thu, 12 Apr 2018 15:02:50 +0100

> A misconfigured system (e.g. with all interrupts affinitised to all CPUs)
>  may produce a storm of ARFS steering events.  With the existing sfc ARFS
>  implementation, that could create a backlog of workitems that grinds the
>  system to a halt.  To prevent this, limit the number of workitems that
>  may be in flight for a given SFC device to 8 (EFX_RPS_MAX_IN_FLIGHT), and
>  return EBUSY from our ndo_rx_flow_steer method if the limit is reached.
> Given this limit, also store the workitems in an array of slots within the
>  struct efx_nic, rather than dynamically allocating for each request.
> 
> Signed-off-by: Edward Cree <ecree@...arflare.com>

I don't think this behavior is all that great.

If you really have to queue up these operations because they take a long
time, I think it is better to enter a synchronous mode and sleep once
you hit this in-flight limit of 8.

Either that or make the expiration work smarter when it has lots of events
to process.