netdev - Enabling gro_flush_timeout/napi_defer_hard

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Message-ID: <39fe64b4-84bd-a40c-2638-c61652642302@gmail.com>
Date:   Wed, 10 Mar 2021 21:22:19 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Enabling gro_flush_timeout/napi_defer_hard_irqs per default in a
 driver?

In r8169 standard irq coalescing is disabled per default, and setting
gro_flush_timeout = 20000 and napi_defer_hard_irqs = 1 reduces interrupt
number significantly, e.g. in iperf.
Therefore I wonder whether it would make sense to set reasonable defaults
for both parameters in the driver, similar to drivers enabling irq
coalescing per default. This way users could benefit who are not
experienced enough to tweak sysfs parameters.
Now to the actual question:
Are you aware of any use cases where setting these parameters could
have a negative impact, what would speak for leaving them disabled
per default?