在 2020/11/12 下午8:19, Vlastimil Babka 写道: > 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@xxxxxxxxxxxxxxxxxxxxxxxxxx/ >> >> 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@xxxxxxxxxxxxxxxxx> >> Acked-by: Hugh Dickins <hughd@xxxxxxxxxx> >> Cc: Rong Chen <rong.a.chen@xxxxxxxxx> >> Cc: Hugh Dickins <hughd@xxxxxxxxxx> >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> >> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> >> Cc: Michal Hocko <mhocko@xxxxxxxxxx> >> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx> >> Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx> >> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> >> Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> >> Cc: Tejun Heo <tj@xxxxxxxxxx> >> Cc: linux-kernel@xxxxxxxxxxxxxxx >> Cc: linux-mm@xxxxxxxxx >> Cc: cgroups@xxxxxxxxxxxxxxx > > 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. > Hi Vlastimil, Thanks for comments! Oh, we did talk about the rcu_read_lock which is used to block memcg destroy during locking. and the spin_lock actually includes a rcu_read_lock(). Yes, we could add this comments later. > 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? The lru isolation still is same as old conception, a set actions that take a page from a lru list, and commit_charge do need a isoltion for the page. but the condition of page_memcg could be change since we don't rely on lru isolation for it. The comments could be changed later. > > 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)); TestClearPageLRU could block the page from memcg migration/destory. Thanks Alex > > 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
