fs/dcache.c: add cond_resched() to shrink_dcache_parent()
Greg Thelen [Tue, 30 Apr 2013 22:26:48 +0000 (15:26 -0700)]
commit 421348f1ca0bf17769dee0aed4d991845ae0536d upstream.

Call cond_resched() in shrink_dcache_parent() to maintain interactivity.

Before this patch:

void shrink_dcache_parent(struct dentry * parent)
{
while ((found = select_parent(parent, &dispose)) != 0)
shrink_dentry_list(&dispose);
}

select_parent() populates the dispose list with dentries which
shrink_dentry_list() then deletes.  select_parent() carefully uses
need_resched() to avoid doing too much work at once.  But neither
shrink_dcache_parent() nor its called functions call cond_resched().  So
once need_resched() is set select_parent() will return single dentry
dispose list which is then deleted by shrink_dentry_list().  This is
inefficient when there are a lot of dentry to process.  This can cause
softlockup and hurts interactivity on non preemptable kernels.

This change adds cond_resched() in shrink_dcache_parent().  The benefit
of this is that need_resched() is quickly cleared so that future calls
to select_parent() are able to efficiently return a big batch of dentry.

These additional cond_resched() do not seem to impact performance, at
least for the workload below.

Here is a program which can cause soft lockup if other system activity
sets need_resched().

int main()
{
        struct rlimit rlim;
        int i;
        int f[100000];
        char buf[20];
        struct timeval t1, t2;
        double diff;

        /* cleanup past run */
        system("rm -rf x");

        /* boost nfile rlimit */
        rlim.rlim_cur = 200000;
        rlim.rlim_max = 200000;
        if (setrlimit(RLIMIT_NOFILE, &rlim))
                err(1, "setrlimit");

        /* make directory for files */
        if (mkdir("x", 0700))
                err(1, "mkdir");

        if (gettimeofday(&t1, NULL))
                err(1, "gettimeofday");

        /* populate directory with open files */
        for (i = 0; i < 100000; i++) {
                snprintf(buf, sizeof(buf), "x/%d", i);
                f[i] = open(buf, O_CREAT);
                if (f[i] == -1)
                        err(1, "open");
        }

        /* close some of the files */
        for (i = 0; i < 85000; i++)
                close(f[i]);

        /* unlink all files, even open ones */
        system("rm -rf x");

        if (gettimeofday(&t2, NULL))
                err(1, "gettimeofday");

        diff = (((double)t2.tv_sec * 1000000 + t2.tv_usec) -
                ((double)t1.tv_sec * 1000000 + t1.tv_usec));

        printf("done: %g elapsed\n", diff/1e6);
        return 0;
}

Signed-off-by: Greg Thelen <gthelen@google.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/dcache.c

index f104945..e498de2 100644 (file)
@@ -1238,8 +1238,10 @@ void shrink_dcache_parent(struct dentry * parent)
        LIST_HEAD(dispose);
        int found;
 
-       while ((found = select_parent(parent, &dispose)) != 0)
+       while ((found = select_parent(parent, &dispose)) != 0) {
                shrink_dentry_list(&dispose);
+               cond_resched();
+       }
 }
 EXPORT_SYMBOL(shrink_dcache_parent);