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Date: Wed, 20 May 2015 16:03:53 +0200 From: Michal Hocko <mhocko@...e.cz> To: Mel Gorman <mgorman@...e.de> Cc: Johannes Weiner <hannes@...xchg.org>, Andrew Morton <akpm@...ux-foundation.org>, Tejun Heo <tj@...nel.org>, Linux-CGroups <cgroups@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 1/2] mm, memcg: Try charging a page before setting page up to date On Wed 20-05-15 13:50:44, Mel Gorman wrote: > Historically memcg overhead was high even if memcg was unused. This has > improved a lot but it still showed up in a profile summary as being a > problem. > > /usr/src/linux-4.0-vanilla/mm/memcontrol.c 6.6441 395842 > mem_cgroup_try_charge 2.950% 175781 > __mem_cgroup_count_vm_event 1.431% 85239 > mem_cgroup_page_lruvec 0.456% 27156 > mem_cgroup_commit_charge 0.392% 23342 > uncharge_list 0.323% 19256 > mem_cgroup_update_lru_size 0.278% 16538 > memcg_check_events 0.216% 12858 > mem_cgroup_charge_statistics.isra.22 0.188% 11172 > try_charge 0.150% 8928 > commit_charge 0.141% 8388 > get_mem_cgroup_from_mm 0.121% 7184 > > That is showing that 6.64% of system CPU cycles were in memcontrol.c and > dominated by mem_cgroup_try_charge. The annotation shows that the bulk of > the cost was checking PageSwapCache which is expected to be cache hot but is > very expensive. The problem appears to be that __SetPageUptodate is called > just before the check which is a write barrier. It is required to make sure > struct page and page data is written before the PTE is updated and the data > visible to userspace. memcg charging does not require or need the barrier > but gets unfairly hit with the cost so this patch attempts the charging > before the barrier. Aside from the accidental cost to memcg there is the > added benefit that the barrier is avoided if the page cannot be charged. > When applied the relevant profile summary is as follows. > > /usr/src/linux-4.0-chargefirst-v2r1/mm/memcontrol.c 3.7907 223277 > __mem_cgroup_count_vm_event 1.143% 67312 > mem_cgroup_page_lruvec 0.465% 27403 > mem_cgroup_commit_charge 0.381% 22452 > uncharge_list 0.332% 19543 > mem_cgroup_update_lru_size 0.284% 16704 > get_mem_cgroup_from_mm 0.271% 15952 > mem_cgroup_try_charge 0.237% 13982 > memcg_check_events 0.222% 13058 > mem_cgroup_charge_statistics.isra.22 0.185% 10920 > commit_charge 0.140% 8235 > try_charge 0.131% 7716 > > That brings the overhead down to 3.79% and leaves the memcg fault accounting > to the root cgroup but it's an improvement. The difference in headline > performance of the page fault microbench is marginal as memcg is such a > small component of it. > > pft faults > 4.0.0 4.0.0 > vanilla chargefirst > Hmean faults/cpu-1 1443258.1051 ( 0.00%) 1509075.7561 ( 4.56%) > Hmean faults/cpu-3 1340385.9270 ( 0.00%) 1339160.7113 ( -0.09%) > Hmean faults/cpu-5 875599.0222 ( 0.00%) 874174.1255 ( -0.16%) > Hmean faults/cpu-7 601146.6726 ( 0.00%) 601370.9977 ( 0.04%) > Hmean faults/cpu-8 510728.2754 ( 0.00%) 510598.8214 ( -0.03%) > Hmean faults/sec-1 1432084.7845 ( 0.00%) 1497935.5274 ( 4.60%) > Hmean faults/sec-3 3943818.1437 ( 0.00%) 3941920.1520 ( -0.05%) > Hmean faults/sec-5 3877573.5867 ( 0.00%) 3869385.7553 ( -0.21%) > Hmean faults/sec-7 3991832.0418 ( 0.00%) 3992181.4189 ( 0.01%) > Hmean faults/sec-8 3987189.8167 ( 0.00%) 3986452.2204 ( -0.02%) > > It's only visible at single threaded. The overhead is there for higher > threads but other factors dominate. > > Signed-off-by: Mel Gorman <mgorman@...e.de> Very well spotted and I wouldn't have figured that out from the profiles posted previously! The patch makes sense. I am wondering why do we still have both __SetPageUptodate and SetPageUptodate when they are same. Historically they were slightly different but this is no longer the case. Acked-by: Michal Hocko <mhocko@...e.cz> Thanks! > --- > mm/memory.c | 10 ++++++---- > 1 file changed, 6 insertions(+), 4 deletions(-) > > diff --git a/mm/memory.c b/mm/memory.c > index 97839f5c8c30..80a03628bd77 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -2158,11 +2158,12 @@ gotten: > goto oom; > cow_user_page(new_page, old_page, address, vma); > } > - __SetPageUptodate(new_page); > > if (mem_cgroup_try_charge(new_page, mm, GFP_KERNEL, &memcg)) > goto oom_free_new; > > + __SetPageUptodate(new_page); > + > mmun_start = address & PAGE_MASK; > mmun_end = mmun_start + PAGE_SIZE; > mmu_notifier_invalidate_range_start(mm, mmun_start, mmun_end); > @@ -2594,6 +2595,10 @@ static int do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma, > page = alloc_zeroed_user_highpage_movable(vma, address); > if (!page) > goto oom; > + > + if (mem_cgroup_try_charge(page, mm, GFP_KERNEL, &memcg)) > + goto oom_free_page; > + > /* > * The memory barrier inside __SetPageUptodate makes sure that > * preceeding stores to the page contents become visible before > @@ -2601,9 +2606,6 @@ static int do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma, > */ > __SetPageUptodate(page); > > - if (mem_cgroup_try_charge(page, mm, GFP_KERNEL, &memcg)) > - goto oom_free_page; > - > entry = mk_pte(page, vma->vm_page_prot); > if (vma->vm_flags & VM_WRITE) > entry = pte_mkwrite(pte_mkdirty(entry)); > -- > 2.3.5 > -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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