linux-kernel - Re: [PATCH v2 net-next] net: poll/select low latency socket support

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Message-ID: <51C05FC1.4000201@linux.intel.com>
Date:	Tue, 18 Jun 2013 16:25:21 +0300
From:	Eliezer Tamir <eliezer.tamir@...ux.intel.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	David Miller <davem@...emloft.net>, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org,
	Jesse Brandeburg <jesse.brandeburg@...el.com>,
	Don Skidmore <donald.c.skidmore@...el.com>,
	e1000-devel@...ts.sourceforge.net,
	Willem de Bruijn <willemb@...gle.com>,
	Ben Hutchings <bhutchings@...arflare.com>,
	Andi Kleen <andi@...stfloor.org>, HPA <hpa@...or.com>,
	Eilon Greenstien <eilong@...adcom.com>,
	Or Gerlitz <or.gerlitz@...il.com>,
	Amir Vadai <amirv@...lanox.com>,
	Alex Rosenbaum <alexr@...lanox.com>,
	Avner Ben Hanoch <avnerb@...lanox.com>,
	Or Kehati <ork@...lanox.com>, sockperf-dev@...glegroups.com,
	Eliezer Tamir <eliezer@...ir.org.il>
Subject: Re: [PATCH v2 net-next] net: poll/select low latency socket support

On 18/06/2013 13:25, Eric Dumazet wrote:
> On Tue, 2013-06-18 at 11:58 +0300, Eliezer Tamir wrote:

>> @@ -731,7 +755,11 @@ static inline unsigned int do_pollfd(struct pollfd *pollfd, poll_table *pwait)
>>   			mask = DEFAULT_POLLMASK;
>>   			if (f.file->f_op && f.file->f_op->poll) {
>>   				pwait->_key = pollfd->events|POLLERR|POLLHUP;
>> +				if (try_ll)
>> +					pwait->_key |= POLL_LL;
>
> Well, why enforce POLL_LL ?
>
> Sure we do this for select() as the API doesn't allow us to add the LL
> flag, but poll() can do that.
>
> An application might set POLL_LL flag only on selected fds.

One other thing,
sock_poll() will only ll_poll if the flag was set _and_ the socket has a
none-zero value in sk->sk_ll_usec so you still only poll on sockets
that were enabled for LLS, not on every socket.

-Eliezer
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