Messages in this thread | | | Subject | Re: [PATCH 1/2] mm: vmscan: skip KSM page in direct reclaim if priority is low | From | Yang Shi <> | Date | Thu, 20 Dec 2018 22:33:26 -0800 |
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On 12/20/18 10:04 PM, Hugh Dickins wrote: > On Thu, 20 Dec 2018, Andrew Morton wrote: >> Is anyone interested in reviewing this? Seems somewhat serious. >> Thanks. > Somewhat serious, but no need to rush. > >> From: Yang Shi <yang.shi@linux.alibaba.com> >> Subject: mm: vmscan: skip KSM page in direct reclaim if priority is low >> >> When running a stress test, we occasionally run into the below hang issue: > Artificial load presumably. > >> INFO: task ksmd:205 blocked for more than 360 seconds. >> Tainted: G E 4.9.128-001.ali3000_nightly_20180925_264.alios7.x86_64 #1 > 4.9-stable does not contain Andrea's 4.13 commit 2c653d0ee2ae > ("ksm: introduce ksm_max_page_sharing per page deduplication limit"). > > The patch below is more economical than Andrea's, but I don't think > a second workaround should be added, unless Andrea's is shown to be > insufficient, even with its ksm_max_page_sharing tuned down to suit. > > Yang, please try to reproduce on upstream, or backport Andrea's to > 4.9-stable - thanks.
I believe Andrea's commit could workaround this problem too by limiting the number of sharing pages.
However, IMHO, even though we just have a few hundred pages share one KSM page, it still sounds not worth reclaiming it in direct reclaim in low priority. According to Andrea's commit log, it still takes a few msec to walk the rmap for 256 shared pages.
Thanks, Yang
> > Hugh > >> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. >> ksmd D 0 205 2 0x00000000 >> ffff882fa00418c0 0000000000000000 ffff882fa4b10000 ffff882fbf059d00 >> ffff882fa5bc1800 ffffc900190c7c28 ffffffff81725e58 ffffffff810777c0 >> 00ffc900190c7c88 ffff882fbf059d00 ffffffff8138cc09 ffff882fa4b10000 >> Call Trace: >> [<ffffffff81725e58>] ? __schedule+0x258/0x720 >> [<ffffffff810777c0>] ? do_flush_tlb_all+0x30/0x30 >> [<ffffffff8138cc09>] ? free_cpumask_var+0x9/0x10 >> [<ffffffff81726356>] schedule+0x36/0x80 >> [<ffffffff81729916>] schedule_timeout+0x206/0x4b0 >> [<ffffffff81077d0f>] ? native_flush_tlb_others+0x11f/0x180 >> [<ffffffff8110ca40>] ? ktime_get+0x40/0xb0 >> [<ffffffff81725b6a>] io_schedule_timeout+0xda/0x170 >> [<ffffffff81726c50>] ? bit_wait+0x60/0x60 >> [<ffffffff81726c6b>] bit_wait_io+0x1b/0x60 >> [<ffffffff81726759>] __wait_on_bit_lock+0x59/0xc0 >> [<ffffffff811aff76>] __lock_page+0x86/0xa0 >> [<ffffffff810d53e0>] ? wake_atomic_t_function+0x60/0x60 >> [<ffffffff8121a269>] ksm_scan_thread+0xeb9/0x1430 >> [<ffffffff810d5340>] ? prepare_to_wait_event+0x100/0x100 >> [<ffffffff812193b0>] ? try_to_merge_with_ksm_page+0x850/0x850 >> [<ffffffff810ac226>] kthread+0xe6/0x100 >> [<ffffffff810ac140>] ? kthread_park+0x60/0x60 >> [<ffffffff8172b196>] ret_from_fork+0x46/0x60 >> >> ksmd found a suitable KSM page on the stable tree and is trying to lock >> it. But it is locked by the direct reclaim path which is walking the >> page's rmap to get the number of referenced PTEs. >> >> The KSM page rmap walk needs to iterate all rmap_items of the page and all >> rmap anon_vmas of each rmap_item. So it may take (# rmap_item * # >> children processes) loops. This number of loops might be very large in >> the worst case, and may take a long time. >> >> Typically, direct reclaim will not intend to reclaim too many pages, and >> it is latency sensitive. So it is not worth doing the long ksm page rmap >> walk to reclaim just one page. >> >> Skip KSM pages in direct reclaim if the reclaim priority is low, but still >> try to reclaim KSM pages with high priority. >> >> Link: http://lkml.kernel.org/r/1541618201-120667-1-git-send-email-yang.shi@linux.alibaba.com >> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com> >> Cc: Vlastimil Babka <vbabka@suse.cz> >> Cc: Johannes Weiner <hannes@cmpxchg.org> >> Cc: Hugh Dickins <hughd@google.com> >> Cc: Michal Hocko <mhocko@kernel.org> >> Cc: Andrea Arcangeli <aarcange@redhat.com> >> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> >> --- >> >> mm/vmscan.c | 23 +++++++++++++++++++++-- >> 1 file changed, 21 insertions(+), 2 deletions(-) >> >> --- a/mm/vmscan.c~mm-vmscan-skip-ksm-page-in-direct-reclaim-if-priority-is-low >> +++ a/mm/vmscan.c >> @@ -1260,8 +1260,17 @@ static unsigned long shrink_page_list(st >> } >> } >> >> - if (!force_reclaim) >> - references = page_check_references(page, sc); >> + if (!force_reclaim) { >> + /* >> + * Don't try to reclaim KSM page in direct reclaim if >> + * the priority is not high enough. >> + */ >> + if (PageKsm(page) && !current_is_kswapd() && >> + sc->priority > (DEF_PRIORITY - 2)) >> + references = PAGEREF_KEEP; >> + else >> + references = page_check_references(page, sc); >> + } >> >> switch (references) { >> case PAGEREF_ACTIVATE: >> @@ -2136,6 +2145,16 @@ static void shrink_active_list(unsigned >> } >> } >> >> + /* >> + * Skip KSM page in direct reclaim if priority is not >> + * high enough. >> + */ >> + if (PageKsm(page) && !current_is_kswapd() && >> + sc->priority > (DEF_PRIORITY - 2)) { >> + putback_lru_page(page); >> + continue; >> + } >> + >> if (page_referenced(page, 0, sc->target_mem_cgroup, >> &vm_flags)) { >> nr_rotated += hpage_nr_pages(page); >> _
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