From mboxrd@z Thu Jan 1 00:00:00 1970 From: Johannes Weiner Subject: Re: [PATCH v8 03/10] mm/lru: replace pgdat lru_lock with lruvec lock Date: Tue, 21 Jan 2020 11:00:05 -0500 Message-ID: <20200121160005.GA69293@cmpxchg.org> References: <1579143909-156105-1-git-send-email-alex.shi@linux.alibaba.com> <1579143909-156105-4-git-send-email-alex.shi@linux.alibaba.com> <20200116215222.GA64230@cmpxchg.org> <9ee80b68-a78f-714a-c727-1f6d2b4f87ea@linux.alibaba.com> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=cmpxchg-org.20150623.gappssmtp.com; s=20150623; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:content-transfer-encoding:in-reply-to; bh=RGHqVegSf46OSrC+vq5Pv4eGAqLOtAwhyN30PfSx9gU=; b=zoinFZ0mYrahd5Hl1dR/pT8vt/qq41K+qA8Hu0uEzs6mlFAFNlvnAch12rsopzsna0 MNXC+r7/nPCC9+9CUKJwbajVlDzmdO6Qgu+H3wuh85tZslqyxdccc4VrsLuTqhRfyDyd MGVpXxoWhYOpTRPzk6Z62zIvWWq9vGnpCkB/mRCo54/h0TzeVtku4lgq589YqsBHdkbJ dnHSE4oCmvIZnG4aEQ3l4mcwVPt9P7qoGGUTGBqLcU8WjhYHGZFAjN9MF1ZnenLyrxVD VMWieaMITU5HC9uhCOAsCsHkG3XVRElBFX2UKbnzLw9BhbBEG3q0JF9z3KxmQ6wWDu4Q GiaA== Content-Disposition: inline In-Reply-To: <9ee80b68-a78f-714a-c727-1f6d2b4f87ea-KPsoFbNs7GizrGE5bRqYAgC/G2K4zDHf@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="utf-8" To: Alex Shi Cc: cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org, mgorman-3eNAlZScCAx27rWaFMvyedHuzzzSOjJt@public.gmane.org, tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, hughd-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org, khlebnikov-XoJtRXgx1JseBXzfvpsJ4g@public.gmane.org, daniel.m.jordan-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org, yang.shi-KPsoFbNs7GizrGE5bRqYAgC/G2K4zDHf@public.gmane.org, willy-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org, shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org, Michal Hocko , Vladimir Davydov , Roman Gushchin , Chris Down , Thomas Gleixner , Vlastimil Babka , Qian Cai , Andrey Ryabinin , "Kirill A. Shutemov" , =?iso-8859-1?B?Suly9G1l?= Glisse , Andrea Arcangeli , David Rientjes , Aneesh Ku On Mon, Jan 20, 2020 at 08:58:09PM +0800, Alex Shi wrote: > > > 在 2020/1/17 上午5:52, Johannes Weiner 写道: > > > You simply cannot serialize on page->mem_cgroup->lruvec when > > page->mem_cgroup isn't stable. You need to serialize on the page > > itself, one way or another, to make this work. > > > > > > So here is a crazy idea that may be worth exploring: > > > > Right now, pgdat->lru_lock protects both PageLRU *and* the lruvec's > > linked list. > > > > Can we make PageLRU atomic and use it to stabilize the lru_lock > > instead, and then use the lru_lock only serialize list operations? > > > > Hi Johannes, > > I am trying to figure out the solution of atomic PageLRU, but is > blocked by the following sitations, when PageLRU and lru list was protected > together under lru_lock, the PageLRU could be a indicator if page on lru list > But now seems it can't be the indicator anymore. > Could you give more clues of stabilization usage of PageLRU? There are two types of PageLRU checks: optimistic and deterministic. The check in activate_page() for example is optimistic and the result unstable, but that's okay, because if we miss a page here and there it's not the end of the world. But the check in __activate_page() is deterministic, because we need to be sure before del_page_from_lru_list(). Currently it's made deterministic by testing under the lock: whoever acquires the lock first gets to touch the LRU state. The same can be done with an atomic TestClearPagLRU: whoever clears the flag first gets to touch the LRU state (the lock is then only acquired to not corrupt the linked list, in case somebody adds or removes a different page at the same time). I.e. in my proposal, if you want to get a stable read of PageLRU, you have to clear it atomically. But AFAICS, everybody who currently does need a stable read either already clears it or can easily be converted to clear it and then set it again (like __activate_page and friends). > __page_cache_release/release_pages/compaction __pagevec_lru_add > if (TestClearPageLRU(page)) if (!PageLRU()) > lruvec_lock(); > list_add(); > lruvec_unlock(); > SetPageLRU() //position 1 > lock_page_lruvec_irqsave(page, &flags); > del_page_from_lru_list(page, lruvec, ..); > unlock_page_lruvec_irqrestore(lruvec, flags); > SetPageLRU() //position 2 Hm, that's not how __pagevec_lru_add() looks. In fact, __pagevec_lru_add_fn() has a BUG_ON(PageLRU). That's because only one thread can own the isolation state at a time. If PageLRU is set, only one thread can claim it. Right now, whoever takes the lock first and clears it wins. When we replace it with TestClearPageLRU, it's the same thing: only one thread can win. And you cannot set PageLRU, unless you own it. Either you isolated the page using TestClearPageLRU, or you allocated a new page. So you can have multiple threads trying to isolate a page from the LRU list, hence the atomic testclear. But no two threads should ever be racing to add a page to the LRU list, because only one thread can own the isolation state. With the atomic PageLRU flag, the sequence would be this: __pagevec_lru_add: BUG_ON(PageLRU()) // Caller *must* own the isolation state lruvec_lock() // The lruvec is stable, because changing // page->mem_cgroup requires owning the // isolation state (PageLRU) and we own it list_add() // Linked list protected by lru_lock lruvec_unlock() SetPageLRU() // The page has been added to the linked // list, give up our isolation state. Once // this flag becomes visible, other threads // can isolate the page from the LRU list