svn commit: r290481 - head/sys/kern
Adrian Chadd
adrian at freebsd.org
Fri Nov 13 23:52:46 UTC 2015
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 at 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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