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X-IronPort-AV: E=McAfee;i="6500,9779,10551"; a="314955739" X-IronPort-AV: E=Sophos;i="5.96,218,1665471600"; d="scan'208";a="314955739" Received: from orsmga008.jf.intel.com ([10.7.209.65]) by fmsmga104.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 04 Dec 2022 18:38:59 -0800 X-IronPort-AV: E=McAfee;i="6500,9779,10551"; a="676429267" X-IronPort-AV: E=Sophos;i="5.96,218,1665471600"; d="scan'208";a="676429267" Received: from yhuang6-desk2.sh.intel.com (HELO yhuang6-desk2.ccr.corp.intel.com) ([10.238.208.55]) by orsmga008-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 04 Dec 2022 18:38:55 -0800 From: "Huang, Ying" To: Mina Almasry Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Yang Shi , Yosry Ahmed , weixugc@google.com, fvdl@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v1] [mm-unstable] mm: Fix memcg reclaim on memory tiered systems References: <20221203011120.2361610-1-almasrymina@google.com> Date: Mon, 05 Dec 2022 10:38:06 +0800 In-Reply-To: <20221203011120.2361610-1-almasrymina@google.com> (Mina Almasry's message of "Fri, 2 Dec 2022 17:11:19 -0800") Message-ID: <87lenm1soh.fsf@yhuang6-desk2.ccr.corp.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii X-Rspamd-Queue-Id: 1C7E1140005 X-Stat-Signature: rs9jfaqqtqxbcsautyrt4uej5cb1mz9m X-Rspam-User: X-Spamd-Result: default: False [-1.30 / 9.00]; DMARC_POLICY_ALLOW(-0.50)[intel.com,none]; BAYES_HAM(-0.40)[71.21%]; R_SPF_ALLOW(-0.20)[+ip4:192.55.52.120/32]; R_DKIM_ALLOW(-0.20)[intel.com:s=Intel]; MIME_GOOD(-0.10)[text/plain]; RCVD_NO_TLS_LAST(0.10)[]; FROM_EQ_ENVFROM(0.00)[]; RCPT_COUNT_TWELVE(0.00)[13]; DKIM_TRACE(0.00)[intel.com:+]; MID_RHS_MATCH_FROMTLD(0.00)[]; TO_MATCH_ENVRCPT_SOME(0.00)[]; RCVD_COUNT_THREE(0.00)[3]; FROM_HAS_DN(0.00)[]; MIME_TRACE(0.00)[0:+]; TO_DN_SOME(0.00)[]; ARC_SIGNED(0.00)[hostedemail.com:s=arc-20220608:i=1]; ARC_NA(0.00)[] X-Rspamd-Server: rspam08 X-HE-Tag: 1670207940-230522 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Mina Almasry writes: > commit 3f1509c57b1b ("Revert "mm/vmscan: never demote for memcg > reclaim"") enabled demotion in memcg reclaim, which is the right thing > to do, however, I suspect it introduced a regression in the behavior of > try_to_free_mem_cgroup_pages(). > > The callers of try_to_free_mem_cgroup_pages() expect it to attempt to > reclaim - not demote - nr_pages from the cgroup. I.e. the memory usage > of the cgroup should reduce by nr_pages. The callers expect > try_to_free_mem_cgroup_pages() to also return the number of pages > reclaimed, not demoted. > > However, what try_to_free_mem_cgroup_pages() actually does is it > unconditionally counts demoted pages as reclaimed pages. So in practice > when it is called it will often demote nr_pages and return the number of > demoted pages to the caller. Demoted pages don't lower the memcg usage, > and so I think try_to_free_mem_cgroup_pages() is not actually doing what > the callers want it to do. > > I suspect various things work suboptimally on memory systems or don't > work at all due to this: > > - memory.high enforcement likely doesn't work (it just demotes nr_pages > instead of lowering the memcg usage by nr_pages). > - try_charge_memcg() will keep retrying the charge while > try_to_free_mem_cgroup_pages() is just demoting pages and not actually > making any room for the charge. > - memory.reclaim has a wonky interface. It advertises to the user it > reclaims the provided amount but it will actually demote that amount. > > There may be more effects to this issue. > > To fix these issues I propose shrink_folio_list() to only count pages > demoted from inside of sc->nodemask to outside of sc->nodemask as > 'reclaimed'. > > For callers such as reclaim_high() or try_charge_memcg() that set > sc->nodemask to NULL, try_to_free_mem_cgroup_pages() will try to > actually reclaim nr_pages and return the number of pages reclaimed. No > demoted pages would count towards the nr_pages requirement. > > For callers such as memory_reclaim() that set sc->nodemask, > try_to_free_mem_cgroup_pages() will free nr_pages from that nodemask > with either reclaim or demotion. Have you checked all callers? For example, IIUC, in reclaim_clean_pages_from_list(), although sc.nodemask == NULL, the demoted pages should be counted as reclaimed. How about count both "demoted" and "reclaimed" in struct scan_control, and let callers to determine how to use the number? > Tested this change using memory.reclaim interface. With this change, > > echo "1m" > memory.reclaim > > Will cause freeing of 1m of memory from the cgroup regardless of the > demotions happening inside. > > echo "1m nodes=0" > memory.reclaim Have you tested these tests in the original kernel? If so, whether does the issue you suspected above occurs during testing? Best Regards, Huang, Ying > Will cause freeing of 1m of node 0 by demotion if a demotion target is > available, and by reclaim if no demotion target is available. > > Signed-off-by: Mina Almasry > > --- > > This is developed on top of mm-unstable largely because I need the > memory.reclaim nodes= arg to test it properly. > --- > mm/vmscan.c | 13 ++++++++++++- > 1 file changed, 12 insertions(+), 1 deletion(-) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 2b42ac9ad755..8f6e993b870d 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -1653,6 +1653,7 @@ static unsigned int shrink_folio_list(struct list_head *folio_list, > LIST_HEAD(free_folios); > LIST_HEAD(demote_folios); > unsigned int nr_reclaimed = 0; > + unsigned int nr_demoted = 0; > unsigned int pgactivate = 0; > bool do_demote_pass; > struct swap_iocb *plug = NULL; > @@ -2085,7 +2086,17 @@ static unsigned int shrink_folio_list(struct list_head *folio_list, > /* 'folio_list' is always empty here */ > > /* Migrate folios selected for demotion */ > - nr_reclaimed += demote_folio_list(&demote_folios, pgdat); > + nr_demoted = demote_folio_list(&demote_folios, pgdat); > + > + /* > + * Only count demoted folios as reclaimed if we demoted them from > + * inside of the nodemask to outside of the nodemask, hence reclaiming > + * pages in the nodemask. > + */ > + if (sc->nodemask && node_isset(pgdat->node_id, *sc->nodemask) && > + !node_isset(next_demotion_node(pgdat->node_id), *sc->nodemask)) > + nr_reclaimed += nr_demoted; > + > /* Folios that could not be demoted are still in @demote_folios */ > if (!list_empty(&demote_folios)) { > /* Folios which weren't demoted go back on @folio_list */ > -- > 2.39.0.rc0.267.gcb52ba06e7-goog