On Tue 17-01-17 23:16:09, Vlastimil Babka wrote:
> This is a preparation for the following patch to make review simpler. While
> the primary motivation is a bug fix, this could also save some cycles in the
> fast path.
I cannot say I would be happy about this patch :/ The code is still very
confusing and subtle. I really think we should get rid of
synchronization with the concurrent cpuset/mempolicy updates instead.
Have you considered that instead?
> Signed-off-by: Vlastimil Babka <vba...@suse.cz>
> ---
> mm/page_alloc.c | 46 +++++++++++++++++++++++++---------------------
> 1 file changed, 25 insertions(+), 21 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index dedadb4a779f..bbc3f015f796 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3502,12 +3502,13 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int
> order,
> struct page *page = NULL;
> unsigned int alloc_flags;
> unsigned long did_some_progress;
> - enum compact_priority compact_priority = DEF_COMPACT_PRIORITY;
> + enum compact_priority compact_priority;
> enum compact_result compact_result;
> - int compaction_retries = 0;
> - int no_progress_loops = 0;
> + int compaction_retries;
> + int no_progress_loops;
> unsigned long alloc_start = jiffies;
> unsigned int stall_timeout = 10 * HZ;
> + unsigned int cpuset_mems_cookie;
>
> /*
> * In the slowpath, we sanity check order to avoid ever trying to
> @@ -3528,6 +3529,12 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int
> order,
> (__GFP_ATOMIC|__GFP_DIRECT_RECLAIM)))
> gfp_mask &= ~__GFP_ATOMIC;
>
> +retry_cpuset:
> + compaction_retries = 0;
> + no_progress_loops = 0;
> + compact_priority = DEF_COMPACT_PRIORITY;
> + cpuset_mems_cookie = read_mems_allowed_begin();
> +
> /*
> * The fast path uses conservative alloc_flags to succeed only until
> * kswapd needs to be woken up, and to avoid the cost of setting up
> @@ -3699,6 +3706,15 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int
> order,
> }
>
> nopage:
> + /*
> + * When updating a task's mems_allowed, it is possible to race with
> + * parallel threads in such a way that an allocation can fail while
> + * the mask is being updated. If a page allocation is about to fail,
> + * check if the cpuset changed during allocation and if so, retry.
> + */
> + if (read_mems_allowed_retry(cpuset_mems_cookie))
> + goto retry_cpuset;
> +
> warn_alloc(gfp_mask,
> "page allocation failure: order:%u", order);
> got_pg:
> @@ -3713,7 +3729,6 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int
> order,
> struct zonelist *zonelist, nodemask_t *nodemask)
> {
> struct page *page;
> - unsigned int cpuset_mems_cookie;
> unsigned int alloc_flags = ALLOC_WMARK_LOW;
> gfp_t alloc_mask = gfp_mask; /* The gfp_t that was actually used for
> allocation */
> struct alloc_context ac = {
> @@ -3750,9 +3765,6 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int
> order,
> if (IS_ENABLED(CONFIG_CMA) && ac.migratetype == MIGRATE_MOVABLE)
> alloc_flags |= ALLOC_CMA;
>
> -retry_cpuset:
> - cpuset_mems_cookie = read_mems_allowed_begin();
> -
> /* Dirty zone balancing only done in the fast path */
> ac.spread_dirty_pages = (gfp_mask & __GFP_WRITE);
>
> @@ -3765,6 +3777,10 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int
> order,
> ac.high_zoneidx, ac.nodemask);
> if (!ac.preferred_zoneref->zone) {
> page = NULL;
> + /*
> + * This might be due to race with cpuset_current_mems_allowed
> + * update, so make sure we retry with original nodemask.
> + */
> goto no_zone;
> }
>
> @@ -3787,27 +3803,15 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int
> order,
> * we could end up iterating over non-eligible zones endlessly.
> */
> if (unlikely(ac.nodemask != nodemask)) {
> +no_zone:
> ac.nodemask = nodemask;
> ac.preferred_zoneref = first_zones_zonelist(ac.zonelist,
> ac.high_zoneidx, ac.nodemask);
> - if (!ac.preferred_zoneref->zone)
> - goto no_zone;
> + /* If we have NULL preferred zone, slowpath wll handle that */
> }
>
> page = __alloc_pages_slowpath(alloc_mask, order, &ac);
>
> -no_zone:
> - /*
> - * When updating a task's mems_allowed, it is possible to race with
> - * parallel threads in such a way that an allocation can fail while
> - * the mask is being updated. If a page allocation is about to fail,
> - * check if the cpuset changed during allocation and if so, retry.
> - */
> - if (unlikely(!page && read_mems_allowed_retry(cpuset_mems_cookie))) {
> - alloc_mask = gfp_mask;
> - goto retry_cpuset;
> - }
> -
> out:
> if (memcg_kmem_enabled() && (gfp_mask & __GFP_ACCOUNT) && page &&
> unlikely(memcg_kmem_charge(page, gfp_mask, order) != 0)) {
> --
> 2.11.0
--
Michal Hocko
SUSE Labs
