Call cond_resched() from shrink_dentry_list() to preserve
shrink_dcache_parent() interactivity.
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 a call to cond_resched() in shrink_dentry_list().
The primary 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. A theoretically secondary benefit of this
change is that shrink_dentry_list() is willing to give up the
processor when working on a huge number 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 on a 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 <gthe...@google.com>
---
fs/dcache.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/fs/dcache.c b/fs/dcache.c
index fbfae008..105e973 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -818,6 +818,8 @@ static void shrink_dentry_list(struct list_head *list)
try_prune_one_dentry(dentry);
+ cond_resched();
+
rcu_read_lock();
}
rcu_read_unlock();
--
1.8.1.3
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