Date: Mon, 29 Sep 2014 11:02:20 -0500 From: Bryan Venteicher <bryanv@daemoninthecloset.org> To: John Baldwin <jhb@freebsd.org> Cc: "freebsd-hackers@freebsd.org" <freebsd-hackers@freebsd.org>, jeff@freebsd.org Subject: Re: Change uma_mtx to rwlock Message-ID: <CAMo0n6SF5KUefTigP=QxEaCKeeMf6Mav_S2pEeqob3xmmNn=1w@mail.gmail.com> In-Reply-To: <1458140.gGPpU3NGiG@ralph.baldwin.cx> References: <CAMo0n6Q=P5H3%2BCqr8KjFRVLvWHZfJYnROVe4xF3DmKw95D%2B5zQ@mail.gmail.com> <1458140.gGPpU3NGiG@ralph.baldwin.cx>
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On Mon, Sep 29, 2014 at 10:27 AM, John Baldwin <jhb@freebsd.org> wrote: > On Saturday, September 27, 2014 07:59:47 PM Bryan Venteicher wrote: > > Hi, > > > > I'd appreciate some comments attached patch that changes the uma_mtx to= a > > rwlock. > > > > At $JOB, we have machines with ~400GB RAM, with much of that being > > allocated through UMA zones. We've observed that timeouts were sometime= s > > unexpectedly delayed by a half second or more. We tracked one of the > > reasons for this down to when the page daemon was running, calling > > uma_reclaim() -> zone_foreach(). zone_foreach() holds the uma_mtx while > > zone_drain()'ing each zone. If uma_timeout() fires, it will block on th= e > > uma_mtx when it tries to zone_timeout() each zone. > > The only nit I see is in zone_drain_wait(). It would be nice to not need > the > hack of checking for a read or write lock and just require the one it > actually > needs depending on the callers. > However, checking the code in HEAD, this appears to just be broken. > Specifically, zone_drain_wait() is called in two places: > > void > zone_drain(uma_zone_t zone) > { > > zone_drain_wait(zone, M_NOWAIT); > } > > ... > > > static void > zone_dtor(void *arg, int size, void *udata) > { > ... > mtx_lock(&uma_mtx); > LIST_REMOVE(zone, uz_link); > mtx_unlock(&uma_mtx); > /* > * XXX there are some races here where > * the zone can be drained but zone lock > * released and then refilled before we > * remove it... we dont care for now > */ > zone_drain_wait(zone, M_WAITOK); > ... > } > > Neither one calls it with the uma_mtx locked! This appears to have been > broken since that function was introduced in r187681. > > =E2=80=8BIndeed. I had noticed and mentioned that when I sent this patch to= jeff@ a few months ago: When zone_dtor() calls zone_drain_wait(), should it hold the uma_{mtx= , rwlock}? Can the zone not be in the DRAINING state at this point? Similarly, does the while draining loop in zone_drain_wait() then take the uma_mtx and the zone lock out of order after the msleep().=E2=80=8B =E2=80=8BBut I was just trying to clear out my queue a bit, and hadn't look= ed at the HEAD UMA in awhile, so I was going to double check that later. I think it might be best to first remove the unlock/lock of uma_mtx from > zone_drain_wait() (so it can be MFC'd). That then simplifies that one > part of > your patch (which I think is otherwise fine). > > =E2=80=8BI'll try to get a review started in Phabric =E2=80=8Bsoon. > -- > John Baldwin > _______________________________________________ > freebsd-hackers@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-hackers > To unsubscribe, send any mail to "freebsd-hackers-unsubscribe@freebsd.org= " >
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