svn commit: r290481 - head/sys/kern
Gleb Smirnoff
glebius at FreeBSD.org
Fri Nov 13 22:44:36 UTC 2015
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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