Date: Mon, 05 Sep 2016 11:42:05 -0700 From: John Baldwin <jhb@freebsd.org> To: Mark Johnston <markj@freebsd.org> Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r305368 - head/sys/kern Message-ID: <28893594.Z98fxgFVzW@ralph.baldwin.cx> In-Reply-To: <20160905180917.GC70066@wkstn-mjohnston.west.isilon.com> References: <201609040029.u840Tn5D019584@repo.freebsd.org> <2134943.hUK9aHv09Y@ralph.baldwin.cx> <20160905180917.GC70066@wkstn-mjohnston.west.isilon.com>
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On Monday, September 05, 2016 11:09:17 AM Mark Johnston wrote: > On Mon, Sep 05, 2016 at 10:30:24AM -0700, John Baldwin wrote: > > On Sunday, September 04, 2016 12:29:49 AM Mark Johnston wrote: > > > Author: markj > > > Date: Sun Sep 4 00:29:48 2016 > > > New Revision: 305368 > > > URL: https://svnweb.freebsd.org/changeset/base/305368 > > > > > > Log: > > > Micro-optimize sleepq_signal(). > > > > > > Lift a comparison out of the loop that finds the highest-priority thread > > > on the queue. > > > > > > MFC after: 1 week > > > > Could this safely use TAILQ_FOREACH_FROM? > > Are you suggesting something like this? > > besttd = TAILQ_FIRST(&sq->sq_blocked[queue]); > td = TAILQ_NEXT(besttd, td_slpq); > TAILQ_FOREACH_FROM(td, &sq->sq_blocked[queue], td_slpq) { > ... > > I think that would work, and it avoids visiting the first element > unnecessarily when the queue contains more than one element. If the > queue contains one element, we'd visit it because of > TAILQ_FOREACH_FROM's surprising behaviour of iterating over the entire > queue when the listelem is NULL. I was hoping it was something equivalent to: for (td = TAILQ_NEXT(besttd); td != NULL; td = TAILQ_NEXT(td)) { ... } I guess what we actually want is something like TAILQ_FOREACH_AFTER(): TAILQ_FOREACH_AFTER(td, besttd, &sq->sq_blocked[queue], td_slpq) that assumed bestttd was not NULL and so avoided the whole "scan the whole thing". For now you could perhaps open-code the above loop though? -- John Baldwin
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