Hello,
Thank you for your comments.
I will modify the patch where necessary and resend v2, but first I will
make 100% sure about the lack of synchronization problem, that might
potentially be there as you mentioned in previous mail , and try to
check some numbers for my support.
Sincerely yours,
Mateusz Nosek
On 9/15/2020 4:04 PM, Michal Hocko wrote:
On Tue 15-09-20 15:09:59, Mateusz Nosek wrote:
On 9/14/2020 4:22 PM, Michal Hocko wrote:
On Mon 14-09-20 12:06:54, mateusznosek0@xxxxxxxxx wrote:
From: Mateusz Nosek <mateusznosek0@xxxxxxxxx>
Most operations from '__alloc_pages_may_oom' do not require oom_mutex hold.
Exception is 'out_of_memory'. The patch refactors '__alloc_pages_may_oom'
to reduce critical section size and improve overall system performance.
This is a real slow path. What is the point of optimizing it? Do you
have any numbers?
I agree that as this is the slow path, then the hard, complicated
optimizations are not recommended. In my humble opinion introduced patch is
not complex and does not decrease code readability or maintainability. In a
nutshell I see no drawbacks of applying it.
This is clearly a matter of taste. I do not see a good reason to apply
it TBH. It is a claimed optimization without any numbers to back that
claim. It is also a tricky area so I am usually very careful to touch
this code without a strong reason. Others might feel differently of
course.
[...]
Anyway, I have only now looked closer at the patch...
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b9bd75cacf02..b07f950a5825 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3935,18 +3935,7 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
.order = order,
};
struct page *page;
-
- *did_some_progress = 0;
-
- /*
- * Acquire the oom lock. If that fails, somebody else is
- * making progress for us.
- */
- if (!mutex_trylock(&oom_lock)) {
- *did_some_progress = 1;
- schedule_timeout_uninterruptible(1);
- return NULL;
- }
+ bool success;
/*
* Go through the zonelist yet one more time, keep very high watermark
@@ -3959,14 +3948,17 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
~__GFP_DIRECT_RECLAIM, order,
ALLOC_WMARK_HIGH|ALLOC_CPUSET, ac);
if (page)
- goto out;
+ return page;
+
+ /* Check if somebody else is making progress for us. */
+ *did_some_progress = mutex_is_locked(&oom_lock);
This is not only quite ugly but wrong as well. In general checking for a
lock state is racy unless the lock is taken somewhere up the call chain.
In this particular case it wouldn't be a big deal because an additional
retry (did_some_progress = 1) is not really critical. It would likely be
nicer to be deterministic here and not retry on all the early bailouts
regardless of the lock state.
/* Coredumps can quickly deplete all memory reserves */
if (current->flags & PF_DUMPCORE)
- goto out;
+ return NULL;
/* The OOM killer will not help higher order allocs */
if (order > PAGE_ALLOC_COSTLY_ORDER)
- goto out;
+ return NULL;
/*
* We have already exhausted all our reclaim opportunities without any
* success so it is time to admit defeat. We will skip the OOM killer
@@ -3976,12 +3968,12 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
* The OOM killer may not free memory on a specific node.
*/
if (gfp_mask & (__GFP_RETRY_MAYFAIL | __GFP_THISNODE))
- goto out;
+ return NULL;
/* The OOM killer does not needlessly kill tasks for lowmem */
if (ac->highest_zoneidx < ZONE_NORMAL)
- goto out;
+ return NULL;
if (pm_suspended_storage())
- goto out;
+ return NULL;
/*
* XXX: GFP_NOFS allocations should rather fail than rely on
* other request to make a forward progress.
@@ -3992,8 +3984,20 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
* failures more gracefully we should just bail out here.
*/
+ /*
+ * Acquire the oom lock. If that fails, somebody else is
+ * making progress for us.
+ */
+ if (!mutex_trylock(&oom_lock)) {
+ *did_some_progress = 1;
+ schedule_timeout_uninterruptible(1);
+ return NULL;
+ }
+ success = out_of_memory(&oc);
+ mutex_unlock(&oom_lock);
+
/* Exhausted what can be done so it's blame time */
- if (out_of_memory(&oc) || WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL)) {
+ if (success || WARN_ON_ONCE(gfp_mask & __GFP_NOFAIL)) {
*did_some_progress = 1;
/*
@@ -4004,8 +4008,7 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
page = __alloc_pages_cpuset_fallback(gfp_mask, order,
ALLOC_NO_WATERMARKS, ac);
}
-out:
- mutex_unlock(&oom_lock);
+
return page;
}
--
2.20.1
Sincerely yours,
Mateusz Nosek
