linux-kernel - Re: [PATCH] selftests/net: deflake GRO tests and fix return value and output

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Message-ID: <Z74icGa4rlnNafoW@google.com>
Date: Tue, 25 Feb 2025 12:05:04 -0800
From: Kevin Krakauer <krakauer@...gle.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: davem@...emloft.net, edumazet@...gle.com, horms@...nel.org,
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
	netdev@...r.kernel.org, pabeni@...hat.com, shuah@...nel.org
Subject: Re: [PATCH] selftests/net: deflake GRO tests and fix return value
 and output

On Mon, Feb 24, 2025 at 12:48:30PM -0800, Jakub Kicinski wrote:
> With msec-long deferrals we'll flush due to jiffies change. At least
> that explains a bit. Could you maybe try lower timeouts than 1msec?
> Previously we'd just keep partially-completed packets in GRO for up 
> to 1msec, now we'll delay all packet processing for 1msec, that's a lot.

Results again with each test run 1000 times:

gro_flush_timeout=50us  napi_defer_hard_irqs=1 --> failed to GRO 0 times
gro_flush_timeout=100us napi_defer_hard_irqs=1 --> failed to GRO 0 times

gro_flush_timeout=50us  napi_defer_hard_irqs=0 --> failed to GRO 36 times
gro_flush_timeout=100us napi_defer_hard_irqs=0 --> failed to GRO 46 times

100us with 1 defer seems to work fine and is well below the duration of
a jiffy. So we'll usually be testing the "default" GRO path and only
occasionally the jiffy-update path. I'll make these the numbers in the
revised patch unless someone thinks otherwise.