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SubjectRe: scheduling problem?
On Wed, 3 Jan 2001, Anton Blanchard wrote:

>
> Hi Mike,
>
> > I am seeing (what I believe is;) severe process CPU starvation in
> > 2.4.0-prerelease. At first, I attributed it to semaphore troubles
> > as when I enable semaphore deadlock detection in IKD and set it to
> > 5 seconds, it triggers 100% of the time on nscd when I do sequential
> > I/O (iozone eg). In the meantime, I've done a slew of tracing, and
> > I think the holder of the semaphore I'm timing out on just flat isn't
> > being scheduled so it can release it. In the usual case of nscd, I
> > _think_ it's another nscd holding the semaphore. In no trace can I
> > go back far enough to catch the taker of the semaphore or any user
> > task other than iozone running between __down() time and timeout 5
> > seconds later. (trace buffer covers ~8 seconds of kernel time)
>
> Did this just appear in recent kernels? Maybe bdflush was hiding the
> situation in earlier kernels as it would cause io hogs to block when
> things got only mildly interesting.

Yes and no. I've seen nasty stalls for quite a while now. (I think
that there is a wakeup problem lurking)

I found the change which triggers my horrid stalls. Nobody is going
to believe this...

diff -urN linux-2.4.0-test13-pre6/fs/buffer.c linux-2.4.0-test13-pre7/fs/buffer.c
--- linux-2.4.0-test13-pre6/fs/buffer.c Sat Dec 30 08:58:56 2000
+++ linux-2.4.0-test13-pre7/fs/buffer.c Sun Dec 31 06:22:31 2000
@@ -122,16 +122,17 @@
when trying to refill buffers. */
int interval; /* jiffies delay between kupdate flushes */
int age_buffer; /* Time for normal buffer to age before we flush it */
- int dummy1; /* unused, was age_super */
+ int nfract_sync; /* Percentage of buffer cache dirty to
+ activate bdflush synchronously */
int dummy2; /* unused */
int dummy3; /* unused */
} b_un;
unsigned int data[N_PARAM];
-} bdf_prm = {{40, 500, 64, 256, 5*HZ, 30*HZ, 5*HZ, 1884, 2}};
+} bdf_prm = {{40, 500, 64, 256, 5*HZ, 30*HZ, 80, 0, 0}};

/* These are the min and max parameter values that we will allow to be assigned */
-int bdflush_min[N_PARAM] = { 0, 10, 5, 25, 0, 1*HZ, 1*HZ, 1, 1};
-int bdflush_max[N_PARAM] = {100,50000, 20000, 20000,600*HZ, 6000*HZ, 6000*HZ, 2047, 5};
+int bdflush_min[N_PARAM] = { 0, 10, 5, 25, 0, 1*HZ, 0, 0, 0};
+int bdflush_max[N_PARAM] = {100,50000, 20000, 20000,600*HZ, 6000*HZ, 100, 0, 0};

/*
* Rewrote the wait-routines to use the "new" wait-queue functionality,
@@ -1032,9 +1034,9 @@
dirty = size_buffers_type[BUF_DIRTY] >> PAGE_SHIFT;
tot = nr_free_buffer_pages();

- dirty *= 200;
+ dirty *= 100;
soft_dirty_limit = tot * bdf_prm.b_un.nfract;
- hard_dirty_limit = soft_dirty_limit * 2;
+ hard_dirty_limit = tot * bdf_prm.b_un.nfract_sync;

/* First, check for the "real" dirty limit. */
if (dirty > soft_dirty_limit) {
...but reversing this cures my semaphore timeouts. Don't say impossible
:) I didn't believe it either until I retested several times. I wager
that if I just fiddle with parameters I'll be able to make the problem
come and go at will. (means the real problem is gonna be a weird one:)

-Mike

-
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