From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mgamail.intel.com (mgamail.intel.com [192.198.163.11]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E6B7EDF51; Thu, 3 Oct 2024 00:35:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=192.198.163.11 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1727915735; cv=none; b=KzDXbYqnsdA0kls4ye+WHhPG68huDdfXKGLxmAECZLgSddd6k6uGuCuY3fYLqmtJ0XacWR2P/GgXsj7bweVrDXRW02uxKs0kL1aSVHkIqkP24WHqEEVvRwtcYGL8VrzyEe9xpQjznon9X7WEsdQsF2kBa4ZNSzL31ZLPCDvNNi0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1727915735; c=relaxed/simple; bh=9Z63A4UwMPg7h2BjcWTLocAQBrgxW8vXstQE/Guqqi8=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=XqjDddlYFTuy7Ufyr0wAWOBc1flZ+IgoksZwABQ2bHYjwAjBRhS0fPj9cXvj+JtMx4ATjZKyJ8BBIy3PHas51ndyELoizqjIiVHkXClhEVkxrfygzg85FlvNcdcgFsxyDstAaBEimGsTWbBaDM9QLkJbhxL6mCBGmUHYBXcGzF8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=intel.com; spf=pass smtp.mailfrom=intel.com; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b=JWOU9aCi; arc=none smtp.client-ip=192.198.163.11 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=intel.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=intel.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b="JWOU9aCi" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1727915733; x=1759451733; h=from:to:cc:subject:in-reply-to:references:date: message-id:mime-version:content-transfer-encoding; bh=9Z63A4UwMPg7h2BjcWTLocAQBrgxW8vXstQE/Guqqi8=; b=JWOU9aCi48vlB6EtbQS51QWbLB+NOsXV49EltCou+DpwIMj9+8OVktTv DU6hqs7H3HdaQkalO8UheyCmxCQn+ckhEmXG2O6iXQrtEpbotbZ7jPx/b fCiY19nfgHu2BkgQsOBuU2qWJ6DlbcSymT9/8B40mw7Hr4AgZZ5vJFYr2 TnL76SExADIDtOrvZ6urboky+TI1OcgboCDhZhUhssknoxcKNQVm/+gGX 6Q2PBLR5wSJVb/Jq+DjKVe8uNJHUHc2n3mrFB2AhUjrcNiPq4qhdD6YyA UZ7QDsg5/KlzFGCiXiqpW/cW7xfdqtDtqesNKJGP6L8zMKuzu/8XwwTEN g==; X-CSE-ConnectionGUID: fpaMjLVARoeeg/UjyH3mAQ== X-CSE-MsgGUID: BrPkpwc6TmKbRbWNR4bLAA== X-IronPort-AV: E=McAfee;i="6700,10204,11213"; a="37694043" X-IronPort-AV: E=Sophos;i="6.11,173,1725346800"; d="scan'208";a="37694043" Received: from fmviesa007.fm.intel.com ([10.60.135.147]) by fmvoesa105.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 02 Oct 2024 17:35:23 -0700 X-CSE-ConnectionGUID: z1ZzhR+/RTubqKTYjKGlDg== X-CSE-MsgGUID: qKKNOiHpRLOzWaZqqA8d4w== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.11,173,1725346800"; d="scan'208";a="73859316" Received: from yhuang6-desk2.sh.intel.com (HELO yhuang6-desk2.ccr.corp.intel.com) ([10.238.208.55]) by fmviesa007-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 02 Oct 2024 17:35:13 -0700 From: "Huang, Ying" To: Barry Song <21cnbao@gmail.com> Cc: akpm@linux-foundation.org, chrisl@kernel.org, david@redhat.com, hannes@cmpxchg.org, hughd@google.com, kaleshsingh@google.com, kasong@tencent.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, liyangouwen1@oppo.com, mhocko@suse.com, minchan@kernel.org, sj@kernel.org, stable@vger.kernel.org, surenb@google.com, v-songbaohua@oppo.com, willy@infradead.org, yosryahmed@google.com, yuzhao@google.com Subject: Re: [PATCH] mm: avoid unconditional one-tick sleep when swapcache_prepare fails In-Reply-To: <20241002015754.969-1-21cnbao@gmail.com> (Barry Song's message of "Wed, 2 Oct 2024 09:57:54 +0800") References: <87y137nxqs.fsf@yhuang6-desk2.ccr.corp.intel.com> <20241002015754.969-1-21cnbao@gmail.com> Date: Thu, 03 Oct 2024 08:31:40 +0800 Message-ID: <87ikuani1f.fsf@yhuang6-desk2.ccr.corp.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Barry Song <21cnbao@gmail.com> writes: > On Wed, Oct 2, 2024 at 8:43=E2=80=AFAM Huang, Ying = wrote: >> >> Barry Song <21cnbao@gmail.com> writes: >> >> > On Tue, Oct 1, 2024 at 7:43=E2=80=AFAM Huang, Ying wrote: >> >> >> >> Barry Song <21cnbao@gmail.com> writes: >> >> >> >> > On Sun, Sep 29, 2024 at 3:43=E2=80=AFPM Huang, Ying wrote: >> >> >> >> >> >> Hi, Barry, >> >> >> >> >> >> Barry Song <21cnbao@gmail.com> writes: >> >> >> >> >> >> > From: Barry Song >> >> >> > >> >> >> > Commit 13ddaf26be32 ("mm/swap: fix race when skipping swapcache") >> >> >> > introduced an unconditional one-tick sleep when `swapcache_prepa= re()` >> >> >> > fails, which has led to reports of UI stuttering on latency-sens= itive >> >> >> > Android devices. To address this, we can use a waitqueue to wake= up >> >> >> > tasks that fail `swapcache_prepare()` sooner, instead of always >> >> >> > sleeping for a full tick. While tasks may occasionally be woken = by an >> >> >> > unrelated `do_swap_page()`, this method is preferable to two sce= narios: >> >> >> > rapid re-entry into page faults, which can cause livelocks, and >> >> >> > multiple millisecond sleeps, which visibly degrade user experien= ce. >> >> >> >> >> >> In general, I think that this works. =C2=A0Why not extend the solu= tion to >> >> >> cover schedule_timeout_uninterruptible() in __read_swap_cache_asyn= c() >> >> >> too? =C2=A0We can call wake_up() when we clear SWAP_HAS_CACHE. =C2= =A0To avoid >> >> > >> >> > Hi Ying, >> >> > Thanks for your comments. >> >> > I feel extending the solution to __read_swap_cache_async() should b= e done >> >> > in a separate patch. On phones, I've never encountered any issues r= eported >> >> > on that path, so it might be better suited for an optimization rath= er than a >> >> > hotfix? >> >> >> >> Yes. =C2=A0It's fine to do that in another patch as optimization. >> > >> > Ok. I'll prepare a separate patch for optimizing that path. >> >> Thanks! >> >> >> >> >> >> overhead to call wake_up() when there's no task waiting, we can us= e an >> >> >> atomic to count waiting tasks. >> >> > >> >> > I'm not sure it's worth adding the complexity, as wake_up() on an e= mpty >> >> > waitqueue should have a very low cost on its own? >> >> >> >> wake_up() needs to call spin_lock_irqsave() unconditionally on a glob= al >> >> shared lock. =C2=A0On systems with many CPUs (such servers), this may= cause >> >> severe lock contention. =C2=A0Even the cache ping-pong may hurt perfo= rmance >> >> much. >> > >> > I understand that cache synchronization was a significant issue before >> > qspinlock, but it seems to be less of a concern after its implementati= on. >> >> Unfortunately, qspinlock cannot eliminate cache ping-pong issue, as >> discussed in the following thread. >> >> https://lore.kernel.org/lkml/20220510192708.GQ76023@worktop.programming.= kicks-ass.net/ >> >> > However, using a global atomic variable would still trigger cache broa= dcasts, >> > correct? >> >> We can only change the atomic variable to non-zero when >> swapcache_prepare() returns non-zero, and call wake_up() when the atomic >> variable is non-zero. =C2=A0Because swapcache_prepare() returns 0 most t= imes, >> the atomic variable is 0 most times. =C2=A0If we don't change the value = of >> atomic variable, cache ping-pong will not be triggered. > > yes. this can be implemented by adding another atomic variable. Just realized that we don't need another atomic variable for this, just use waitqueue_active() before wake_up() should be enough. >> >> Hi, Kairui, >> >> Do you have some test cases to test parallel zram swap-in? =C2=A0If so, = that >> can be used to verify whether cache ping-pong is an issue and whether it >> can be fixed via a global atomic variable. >> > > Yes, Kairui please run a test on your machine with lots of cores before > and after adding a global atomic variable as suggested by Ying. I am > sorry I don't have a server machine. > > if it turns out you find cache ping-pong can be an issue, another > approach would be a waitqueue hash: Yes. waitqueue hash may help reduce lock contention. And, we can have both waitqueue_active() and waitqueue hash if necessary. As the first step, waitqueue_active() appears simpler. > diff --git a/mm/memory.c b/mm/memory.c > index 2366578015ad..aae0e532d8b6 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -4192,6 +4192,23 @@ static struct folio *alloc_swap_folio(struct vm_fa= ult *vmf) > } > #endif /* CONFIG_TRANSPARENT_HUGEPAGE */ >=20=20 > +/* > + * Alleviating the 'thundering herd' phenomenon using a waitqueue hash > + * when multiple do_swap_page() operations occur simultaneously. > + */ > +#define SWAPCACHE_WAIT_TABLE_BITS 5 > +#define SWAPCACHE_WAIT_TABLE_SIZE (1 << SWAPCACHE_WAIT_TABLE_BITS) > +static wait_queue_head_t swapcache_wqs[SWAPCACHE_WAIT_TABLE_SIZE]; > + > +static int __init swapcache_wqs_init(void) > +{ > + for (int i =3D 0; i < SWAPCACHE_WAIT_TABLE_SIZE; i++) > + init_waitqueue_head(&swapcache_wqs[i]); > + > + return 0; > +} > +late_initcall(swapcache_wqs_init); > + > /* > * We enter with non-exclusive mmap_lock (to exclude vma changes, > * but allow concurrent faults), and pte mapped but not yet locked. > @@ -4204,6 +4221,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > { > struct vm_area_struct *vma =3D vmf->vma; > struct folio *swapcache, *folio =3D NULL; > + DECLARE_WAITQUEUE(wait, current); > + wait_queue_head_t *swapcache_wq; > struct page *page; > struct swap_info_struct *si =3D NULL; > rmap_t rmap_flags =3D RMAP_NONE; > @@ -4297,12 +4316,16 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > * undetectable as pte_same() returns true due > * to entry reuse. > */ > + swapcache_wq =3D &swapcache_wqs[hash_long(vmf->address & PMD_MASK, > + SWAPCACHE_WAIT_TABLE_BITS)]; > if (swapcache_prepare(entry, nr_pages)) { > /* > * Relax a bit to prevent rapid > * repeated page faults. > */ > + add_wait_queue(swapcache_wq, &wait); > schedule_timeout_uninterruptible(1); > + remove_wait_queue(swapcache_wq, &wait); > goto out_page; > } > need_clear_cache =3D true; > @@ -4609,8 +4632,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > pte_unmap_unlock(vmf->pte, vmf->ptl); > out: > /* Clear the swap cache pin for direct swapin after PTL unlock */ > - if (need_clear_cache) > + if (need_clear_cache) { > swapcache_clear(si, entry, nr_pages); > + wake_up(swapcache_wq); > + } > if (si) > put_swap_device(si); > return ret; > @@ -4625,8 +4650,10 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > folio_unlock(swapcache); > folio_put(swapcache); > } > - if (need_clear_cache) > + if (need_clear_cache) { > swapcache_clear(si, entry, nr_pages); > + wake_up(swapcache_wq); > + } > if (si) > put_swap_device(si); > return ret; -- Best Regards, Huang, Ying