netdev - RE: [RFC 0/7] Add support to process rx packets in thread

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Date:   Tue, 28 Jul 2020 22:29:02 +0530
From:   "Rakesh Pillai" <pillair@...eaurora.org>
To:     "'David Laight'" <David.Laight@...LAB.COM>,
        "'Sebastian Gottschall'" <s.gottschall@...wrt.com>,
        "'Hillf Danton'" <hdanton@...a.com>
Cc:     "'Andrew Lunn'" <andrew@...n.ch>, <netdev@...r.kernel.org>,
        <linux-wireless@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <ath10k@...ts.infradead.org>, <dianders@...omium.org>,
        "'Markus Elfring'" <Markus.Elfring@....de>, <evgreen@...omium.org>,
        <kuba@...nel.org>, <johannes@...solutions.net>,
        <davem@...emloft.net>, <kvalo@...eaurora.org>
Subject: RE: [RFC 0/7] Add support to process rx packets in thread



> -----Original Message-----
> From: David Laight <David.Laight@...LAB.COM>
> Sent: Sunday, July 26, 2020 4:46 PM
> To: 'Sebastian Gottschall' <s.gottschall@...wrt.com>; Hillf Danton
> <hdanton@...a.com>
> Cc: Andrew Lunn <andrew@...n.ch>; Rakesh Pillai <pillair@...eaurora.org>;
> netdev@...r.kernel.org; linux-wireless@...r.kernel.org; linux-
> kernel@...r.kernel.org; ath10k@...ts.infradead.org;
> dianders@...omium.org; Markus Elfring <Markus.Elfring@....de>;
> evgreen@...omium.org; kuba@...nel.org; johannes@...solutions.net;
> davem@...emloft.net; kvalo@...eaurora.org
> Subject: RE: [RFC 0/7] Add support to process rx packets in thread
> 
> From: Sebastian Gottschall <s.gottschall@...wrt.com>
> > 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.
> 
> 	David
> 

Hi All,

Is the threaded NAPI change posted to kernel ? 
Is the conclusion of this discussion that " we cannot use threads for processing packets " ??


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