From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alex Shi Subject: Re: [PATCH v17 17/21] mm/lru: replace pgdat lru_lock with lruvec lock Date: Wed, 29 Jul 2020 10:27:34 +0800 Message-ID: <0590862b-0705-fb9b-be1b-ed0745ca1b76@linux.alibaba.com> References: <1595681998-19193-1-git-send-email-alex.shi@linux.alibaba.com> <1595681998-19193-18-git-send-email-alex.shi@linux.alibaba.com> <09aeced7-cc36-0c9a-d40b-451db9dc54cc@linux.alibaba.com> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="utf-8" To: Alexander Duyck Cc: Andrew Morton , Mel Gorman , Tejun Heo , Hugh Dickins , Konstantin Khlebnikov , Daniel Jordan , Yang Shi , Matthew Wilcox , Johannes Weiner , kbuild test robot , linux-mm , LKML , cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Shakeel Butt , Joonsoo Kim , Wei Yang , "Kirill A. Shutemov" , Rong Chen , Michal Hocko , Vladimir Davydov 在 2020/7/29 上午9:27, Alexander Duyck 写道: > On Tue, Jul 28, 2020 at 6:00 PM Alex Shi wrote: >> >> >> >> 在 2020/7/28 下午10:54, Alexander Duyck 写道: >>> On Tue, Jul 28, 2020 at 4:20 AM Alex Shi wrote: >>>> >>>> >>>> >>>> 在 2020/7/28 上午7:34, Alexander Duyck 写道: >>>>>> @@ -1876,6 +1876,12 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec, >>>>>> * list_add(&page->lru,) >>>>>> * list_add(&page->lru,) //corrupt >>>>>> */ >>>>>> + new_lruvec = mem_cgroup_page_lruvec(page, page_pgdat(page)); >>>>>> + if (new_lruvec != lruvec) { >>>>>> + if (lruvec) >>>>>> + spin_unlock_irq(&lruvec->lru_lock); >>>>>> + lruvec = lock_page_lruvec_irq(page); >>>>>> + } >>>>>> SetPageLRU(page); >>>>>> >>>>>> if (unlikely(put_page_testzero(page))) { >>>>> I was going through the code of the entire patch set and I noticed >>>>> these changes in move_pages_to_lru. What is the reason for adding the >>>>> new_lruvec logic? My understanding is that we are moving the pages to >>>>> the lruvec provided are we not?If so why do we need to add code to get >>>>> a new lruvec? The code itself seems to stand out from the rest of the >>>>> patch as it is introducing new code instead of replacing existing >>>>> locking code, and it doesn't match up with the description of what >>>>> this function is supposed to do since it changes the lruvec. >>>> >>>> this new_lruvec is the replacement of removed line, as following code: >>>>>> - lruvec = mem_cgroup_page_lruvec(page, pgdat); >>>> This recheck is for the page move the root memcg, otherwise it cause the bug: >>> >>> Okay, now I see where the issue is. You moved this code so now it has >>> a different effect than it did before. You are relocking things before >>> you needed to. Don't forget that when you came into this function you >>> already had the lock. In addition the patch is broken as it currently >>> stands as you aren't using similar logic in the code just above this >>> addition if you encounter an evictable page. As a result this is >>> really difficult to review as there are subtle bugs here. >> >> Why you think its a bug? the relock only happens if locked lruvec is different. >> and unlock the old one. > > The section I am talking about with the bug is this section here: > while (!list_empty(list)) { > + struct lruvec *new_lruvec = NULL; > + > page = lru_to_page(list); > VM_BUG_ON_PAGE(PageLRU(page), page); > list_del(&page->lru); > if (unlikely(!page_evictable(page))) { > - spin_unlock_irq(&pgdat->lru_lock); > + spin_unlock_irq(&lruvec->lru_lock); > putback_lru_page(page); > - spin_lock_irq(&pgdat->lru_lock); > + spin_lock_irq(&lruvec->lru_lock); It would be still fine. The lruvec->lru_lock will be checked again before we take and use it. And this lock will optimized in patch 19th which did by Hugh Dickins. > continue; > } > > Basically it probably is not advisable to be retaking the > lruvec->lru_lock directly as the lruvec may have changed so it > wouldn't be correct for the next page. It would make more sense to be > using your API and calling unlock_page_lruvec_irq and > lock_page_lruvec_irq instead of using the lock directly. > >>> >>> I suppose the correct fix is to get rid of this line, but it should >>> be placed everywhere the original function was calling >>> spin_lock_irq(). >>> >>> In addition I would consider changing the arguments/documentation for >>> move_pages_to_lru. You aren't moving the pages to lruvec, so there is >>> probably no need to pass that as an argument. Instead I would pass >>> pgdat since that isn't going to be moving and is the only thing you >>> actually derive based on the original lruvec. >> >> yes, The comments should be changed with the line was introduced from long ago. :) >> Anyway, I am wondering if it worth a v18 version resend? > > So I have been looking over the function itself and I wonder if it > isn't worth looking at rewriting this to optimize the locking behavior > to minimize the number of times we have to take the LRU lock. I have > some code I am working on that I plan to submit as an RFC in the next > day or so after I can get it smoke tested. The basic idea would be to > defer returning the evictiable pages or freeing the compound pages > until after we have processed the pages that can be moved while still > holding the lock. I would think it should reduce the lock contention > significantly while improving the throughput. > I had tried once, but the freeing page cross onto release_pages which hard to deal with. I am very glad to wait your patch, and hope it could be resovled. :) Thanks Alex