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

From: "Rakesh Pillai" <pillair@codeaurora.org>
To: "'David Laight'" <David.Laight@ACULAB.COM>,
	"'Sebastian Gottschall'" <s.gottschall@dd-wrt.com>,
	"'Hillf Danton'" <hdanton@sina.com>
Cc: "'Andrew Lunn'" <andrew@lunn.ch>, <netdev@vger.kernel.org>,
	<linux-wireless@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
	<ath10k@lists.infradead.org>, <dianders@chromium.org>,
	"'Markus Elfring'" <Markus.Elfring@web.de>,
	<evgreen@chromium.org>, <kuba@kernel.org>,
	<johannes@sipsolutions.net>, <davem@davemloft.net>,
	<kvalo@codeaurora.org>
Subject: RE: [RFC 0/7] Add support to process rx packets in thread
Date: Tue, 28 Jul 2020 22:29:02 +0530	[thread overview]
Message-ID: <001001d66500$69a58970$3cf09c50$@codeaurora.org> (raw)
In-Reply-To: <cb54c2746a3d4ce695e3bda8b576b40e@AcuMS.aculab.com>

> -----Original Message-----
> From: David Laight <David.Laight@ACULAB.COM>
> Sent: Sunday, July 26, 2020 4:46 PM
> To: 'Sebastian Gottschall' <s.gottschall@dd-wrt.com>; Hillf Danton
> <hdanton@sina.com>
> Cc: Andrew Lunn <andrew@lunn.ch>; Rakesh Pillai <pillair@codeaurora.org>;
> netdev@vger.kernel.org; linux-wireless@vger.kernel.org; linux-
> kernel@vger.kernel.org; ath10k@lists.infradead.org;
> dianders@chromium.org; Markus Elfring <Markus.Elfring@web.de>;
> evgreen@chromium.org; kuba@kernel.org; johannes@sipsolutions.net;
> davem@davemloft.net; kvalo@codeaurora.org
> Subject: RE: [RFC 0/7] Add support to process rx packets in thread
> 
> From: Sebastian Gottschall <s.gottschall@dd-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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