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Date:   Sun, 26 Jul 2020 11:16:00 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Sebastian Gottschall' <>,
        Hillf Danton <>
CC:     Andrew Lunn <>,
        Rakesh Pillai <>,
        "" <>,
        "" <>,
        "" <>,
        "" <>,
        "" <>,
        Markus Elfring <>,
        "" <>,
        "" <>,
        "" <>,
        "" <>,
        "" <>
Subject: RE: [RFC 0/7] Add support to process rx packets in thread

From: Sebastian Gottschall <>
> Sent: 25 July 2020 16:42
> >> i agree. i just can say that i tested this patch recently due this
> >> discussion here. and it can be changed by sysfs. but it doesnt work for
> >> wifi drivers which are mainly using dummy netdev devices. for this i
> >> made a small patch to get them working using napi_set_threaded manually
> >> hardcoded in the drivers. (see patch bellow)

> > By CONFIG_THREADED_NAPI, there is no need to consider what you did here
> > in the napi core because device drivers know better and are responsible
> > for it before calling napi_schedule(n).

> yeah. but that approach will not work for some cases. some stupid
> drivers are using locking context in the napi poll function.
> in that case the performance will runto shit. i discovered this with the
> mvneta eth driver (marvell) and mt76 tx polling (rx  works)
> for mvneta is will cause very high latencies and packet drops. for mt76
> it causes packet stop. doesnt work simply (on all cases no crashes)
> so the threading will only work for drivers which are compatible with
> that approach. it cannot be used as drop in replacement from my point of
> view.
> its all a question of the driver design

Why should it make (much) difference whether the napi callbacks (etc)
are done in the context of the interrupted process or that of a
specific kernel thread.
The process flags (or whatever) can even be set so that it appears
to be the expected 'softint' context.

In any case running NAPI from a thread will just show up the next
piece of code that runs for ages in softint context.
I think I've seen the tail end of memory being freed under rcu
finally happening under softint and taking absolutely ages.


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