Re: [PATCH v9 net-next 5/7] net: simple poll/select low latencysocket poll

From: Eric Dumazet
Date: Wed Jun 05 2013 - 09:57:01 EST

On Wed, 2013-06-05 at 16:41 +0300, Eliezer Tamir wrote:
> On 05/06/2013 16:30, Eric Dumazet wrote:
> > On Wed, 2013-06-05 at 13:34 +0300, Eliezer Tamir wrote:
> >> A very naive select/poll busy-poll support.
> >> Add busy-polling to sock_poll().
> >> When poll/select have nothing to report, call the low-level
> >> sock_poll() again until we are out of time or we find something.
> >> Right now we poll every socket once, this is suboptimal
> >> but improves latency when the number of sockets polled is not large.
> >>
> >> Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxx>
> >> Signed-off-by: Jesse Brandeburg <jesse.brandeburg@xxxxxxxxx>
> >> Tested-by: Willem de Bruijn <willemb@xxxxxxxxxx>
> >> Signed-off-by: Eliezer Tamir <eliezer.tamir@xxxxxxxxxxxxxxx>
> >> ---
> >
> > I am a bit uneasy with this one, because an applicatio polling() on one
> > thousand file descriptors using select()/poll(), will call sk_poll_ll()
> > one thousand times.
> But we call sk_poll_ll() with nonblock set, so it will only test once
> for each socket and not loop.
> I think this is not as bad as it sounds.
> We still honor the time limit on how long to poll.
> When we busy-wait on a single socket we call sk_poll_ll() repeatedly
> until we timeout or we have something to report.
> Here on the other hand, we sk_poll_ll() once for each file, so we loop
> on the files. We moved the loop from inside sk_poll_ll to select/poll.
> I also plan on improving this this in the next stage.
> The plan is to give control on whether sk_poll_ll is called to
> select/poll/epoll, so the caller has even more control.

This looks quite easy, by adding in include/uapi/asm-generic/poll.h

#define POLL_LL 0x8000

And do the sk_poll_ll() call only if flag is set.

I do not think we have to support select(), as its legacy interface, and
people wanting ll should really use epoll() or poll().

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