[069/118] fs/dcache.c: add cond_resched() to shrink_dcache_parent()

Ben Hutchings
2013-05-10T14:36:09Z

From: Greg Thelen <gthelen@google.com>

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: Ben Hutchings <ben@decadent.org.uk>
---
fs/dcache.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1176,8 +1176,10 @@ void shrink_dcache_parent(struct dentry
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);