Date: Fri, 13 Nov 2015 15:52:45 -0800 From: Adrian Chadd <adrian@freebsd.org> To: Gleb Smirnoff <glebius@freebsd.org> Cc: "src-committers@freebsd.org" <src-committers@freebsd.org>, "svn-src-all@freebsd.org" <svn-src-all@freebsd.org>, "svn-src-head@freebsd.org" <svn-src-head@freebsd.org> Subject: Re: svn commit: r290481 - head/sys/kern Message-ID: <CAJ-Vmo=5z7ZP1HPPX78GYg2NnwFZ8PJF3Z31b7B1CLYb40H-Gg@mail.gmail.com> In-Reply-To: <20151113224431.GD73031@FreeBSD.org> References: <201511070404.tA7440RE080849@repo.freebsd.org> <20151113224431.GD73031@FreeBSD.org>
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Peter Holm already approached me to ask about it. The problem has been figuring out how to trigger it in an automated / VM way. :( -a On 13 November 2015 at 14:44, Gleb Smirnoff <glebius@freebsd.org> wrote: > On Sat, Nov 07, 2015 at 04:04:00AM +0000, Adrian Chadd wrote: > A> Author: adrian > A> Date: Sat Nov 7 04:04:00 2015 > A> New Revision: 290481 > A> URL: https://svnweb.freebsd.org/changeset/base/290481 > A> > A> Log: > A> Add a sched_yield() to work around low memory conditions in the current code. > A> > A> Things seem to get stuck in low memory conditions where no bufs are available, > A> the reclamation path is called to wakeup the daemon, but no sleeping is done. > A> Because of this, we are stuck in a tight loop in the current process and > A> never run said reclamation path. > A> > A> This was introduced in r289279 . This is only a temporary workaround > A> to restore system usefulness until the more permanent solutions can be > A> found. > A> > A> Tested: > A> > A> * Carambola2, 64MB (and 32MB by manual config.) > A> > A> Modified: > A> head/sys/kern/vfs_bio.c > A> > A> Modified: head/sys/kern/vfs_bio.c > A> ============================================================================== > A> --- head/sys/kern/vfs_bio.c Sat Nov 7 02:18:19 2015 (r290480) > A> +++ head/sys/kern/vfs_bio.c Sat Nov 7 04:04:00 2015 (r290481) > A> @@ -3622,6 +3622,23 @@ loop: > A> if (bp == NULL) { > A> if (slpflag || slptimeo) > A> return NULL; > A> + /* > A> + * XXX This is here until the sleep path is diagnosed > A> + * enough to work under very low memory conditions. > A> + * > A> + * There's an issue on low memory, 4BSD+non-preempt > A> + * systems (eg MIPS routers with 32MB RAM) where buffer > A> + * exhaustion occurs without sleeping for buffer > A> + * reclaimation. This just sticks in a loop and > A> + * constantly attempts to allocate a buffer, which > A> + * hits exhaustion and tries to wakeup bufdaemon. > A> + * This never happens because we never yield. > A> + * > A> + * The real solution is to identify and fix these cases > A> + * so we aren't effectively busy-waiting in a loop > A> + * until the reclaimation path has cycles to run. > A> + */ > A> + kern_yield(PRI_USER); > A> goto loop; > A> } > > It'll be nice if such crutches have a test case in src/tests, and a > reference to it in the comment above. So that in future, someone > considering that the crutch is no longer needed, can quickly check that > assumption. > > -- > Totus tuus, Glebius.
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