From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail144.messagelabs.com (mail144.messagelabs.com [216.82.254.51]) by kanga.kvack.org (Postfix) with ESMTP id EF2396B002D for ; Wed, 9 Nov 2011 04:05:54 -0500 (EST) Received: from d28relay03.in.ibm.com (d28relay03.in.ibm.com [9.184.220.60]) by e28smtp03.in.ibm.com (8.14.4/8.13.1) with ESMTP id pA995luu025761 for ; Wed, 9 Nov 2011 14:35:47 +0530 Received: from d28av01.in.ibm.com (d28av01.in.ibm.com [9.184.220.63]) by d28relay03.in.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id pA993DRr4112392 for ; Wed, 9 Nov 2011 14:33:17 +0530 Received: from d28av01.in.ibm.com (loopback [127.0.0.1]) by d28av01.in.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id pA993D3m001188 for ; Wed, 9 Nov 2011 14:33:13 +0530 Message-ID: <4EBA41D1.3020008@linux.vnet.ibm.com> Date: Wed, 09 Nov 2011 14:33:13 +0530 From: "Srivatsa S. Bhat" MIME-Version: 1.0 Subject: Re: [PATCH] thp: reduce khugepaged freezing latency References: <4EB8E969.6010502@suse.cz> <1320766151-2619-1-git-send-email-aarcange@redhat.com> <1320766151-2619-2-git-send-email-aarcange@redhat.com> <4EB98A83.3040101@linux.vnet.ibm.com> <20111109000146.GA5075@redhat.com> In-Reply-To: <20111109000146.GA5075@redhat.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Andrea Arcangeli Cc: "Rafael J. Wysocki" , linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org, Jiri Slaby , linux-mm@kvack.org, Andrew Morton On 11/09/2011 05:31 AM, Andrea Arcangeli wrote: > On Wed, Nov 09, 2011 at 01:31:07AM +0530, Srivatsa S. Bhat wrote: >> On 11/08/2011 08:59 PM, Andrea Arcangeli wrote: >>> Lack of set_freezable_with_signal() prevented khugepaged to be waken >>> up (and prevented to sleep again) across the >>> schedule_timeout_interruptible() calls after freezing() becomes >>> true. The tight loop in khugepaged_alloc_hugepage() also missed one >>> try_to_freeze() call in case alloc_hugepage() would repeatedly fail in >>> turn preventing the loop to break and to reach the try_to_freeze() in >>> the khugepaged main loop. >>> >>> khugepaged would still freeze just fine by trying again the next >>> minute but it's better if it freezes immediately. >>> >>> Reported-by: Jiri Slaby >>> Signed-off-by: Andrea Arcangeli >>> --- >>> mm/huge_memory.c | 3 ++- >>> 1 files changed, 2 insertions(+), 1 deletions(-) >>> >>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>> index 4298aba..67311d1 100644 >>> --- a/mm/huge_memory.c >>> +++ b/mm/huge_memory.c >>> @@ -2277,6 +2277,7 @@ static struct page *khugepaged_alloc_hugepage(void) >>> if (!hpage) { >>> count_vm_event(THP_COLLAPSE_ALLOC_FAILED); >>> khugepaged_alloc_sleep(); >>> + try_to_freeze(); >>> } else >>> count_vm_event(THP_COLLAPSE_ALLOC); >>> } while (unlikely(!hpage) && >>> @@ -2331,7 +2332,7 @@ static int khugepaged(void *none) >>> { >>> struct mm_slot *mm_slot; >>> >>> - set_freezable(); >>> + set_freezable_with_signal(); >>> set_user_nice(current, 19); >>> >>> /* serialize with start_khugepaged() */ >>> >> >> Why do we need to use both set_freezable_with_signal() and an additional >> try_to_freeze()? Won't just using either one of them be good enough? >> Or am I missing something here? > > set_freezable_with_signal() makes khugepaged quit and not re-enter the > sleep, try_to_freeze is needed to get the task from freezing to > frozen, otherwise it'll loop without getting frozen. > Sorry, I still don't get it. Correct me if I am wrong, but my understanding is this: There are 2 ways to freeze a freezable kernel thread (one which has unset the PF_NOFREEZE flag by calling set_freezable()): set TIF_FREEZE flag and, a) send a signal if PF_FREEZER_NOSIG is unset for that kernel thread (due to the call to set_freezable_with_signal()). Then, try_to_freeze() will get called in the signal handler. b) otherwise, just wake up the kernel thread and hope that the kernel thread itself will call try_to_freeze() sometime soon. Now coming to your patch, Case 1: You use set_freezable_with_signal() instead of set_freezable(): In this case, since the kernel thread doesn't block signals for freezing, it will get a signal (with TIF_FREEZE set) and the signal handler will call try_to_freeze(). So, no need for additional try_to_freeze() here. Case 2: You add the extra try_to_freeze(): In this case, the freezer will wake up the kernel thread, which in turn will now execute the newly added try_to_freeze() and will get frozen successfully. So, no need for set_freezable_with_signal() here. Rafael, am I right? Thanks, Srivatsa S. Bhat -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org