Messages in this thread | | | From | Vlastimil Babka <> | Subject | Re: [PATCH v21 17/19] mm/lru: replace pgdat lru_lock with lruvec lock | Date | Thu, 12 Nov 2020 13:19:18 +0100 |
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On 11/5/20 9:55 AM, Alex Shi wrote: > This patch moves per node lru_lock into lruvec, thus bring a lru_lock for > each of memcg per node. So on a large machine, each of memcg don't > have to suffer from per node pgdat->lru_lock competition. They could go > fast with their self lru_lock. > > After move memcg charge before lru inserting, page isolation could > serialize page's memcg, then per memcg lruvec lock is stable and could > replace per node lru lock. > > In func isolate_migratepages_block, compact_unlock_should_abort and > lock_page_lruvec_irqsave are open coded to work with compact_control. > Also add a debug func in locking which may give some clues if there are > sth out of hands. > > Daniel Jordan's testing show 62% improvement on modified readtwice case > on his 2P * 10 core * 2 HT broadwell box. > https://lore.kernel.org/lkml/20200915165807.kpp7uhiw7l3loofu@ca-dmjordan1.us.oracle.com/ > > On a large machine with memcg enabled but not used, the page's lruvec > seeking pass a few pointers, that may lead to lru_lock holding time > increase and a bit regression. > > Hugh Dickins helped on the patch polish, thanks! > > Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com> > Acked-by: Hugh Dickins <hughd@google.com> > Cc: Rong Chen <rong.a.chen@intel.com> > Cc: Hugh Dickins <hughd@google.com> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: Johannes Weiner <hannes@cmpxchg.org> > Cc: Michal Hocko <mhocko@kernel.org> > Cc: Vladimir Davydov <vdavydov.dev@gmail.com> > Cc: Yang Shi <yang.shi@linux.alibaba.com> > Cc: Matthew Wilcox <willy@infradead.org> > Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru> > Cc: Tejun Heo <tj@kernel.org> > Cc: linux-kernel@vger.kernel.org > Cc: linux-mm@kvack.org > Cc: cgroups@vger.kernel.org
I think I need some explanation about the rcu_read_lock() usage in lock_page_lruvec*() (and places effectively opencoding it). Preferably in form of some code comment, but that can be also added as a additional patch later, I don't want to block the series.
mem_cgroup_page_lruvec() comment says
* This function relies on page->mem_cgroup being stable - see the * access rules in commit_charge().
commit_charge() comment:
* Any of the following ensures page->mem_cgroup stability: * * - the page lock * - LRU isolation * - lock_page_memcg() * - exclusive reference
"LRU isolation" used to be quite clear, but now is it after TestClearPageLRU(page) or after deleting from the lru list as well? Also it doesn't mention rcu_read_lock(), should it?
So what exactly are we protecting by rcu_read_lock() in e.g. lock_page_lruvec()?
rcu_read_lock(); lruvec = mem_cgroup_page_lruvec(page, pgdat); spin_lock(&lruvec->lru_lock); rcu_read_unlock();
Looks like we are protecting the lruvec from going away and it can't go away anymore after we take the lru_lock?
But then e.g. in __munlock_pagevec() we are doing this without an rcu_read_lock():
new_lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page));
where new_lruvec is potentionally not the one that we have locked
And the last thing mem_cgroup_page_lruvec() is doing is:
if (unlikely(lruvec->pgdat != pgdat)) lruvec->pgdat = pgdat; return lruvec;
So without the rcu_read_lock() is this potentionally accessing the pgdat field of lruvec that might have just gone away?
Thanks, Vlastimil
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