From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-ej1-f47.google.com (mail-ej1-f47.google.com [209.85.218.47]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 75C7613DBBE for ; Mon, 18 Nov 2024 09:04:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.218.47 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1731920663; cv=none; b=WYRvF9yyg2eLJMWDdHLaaU0toIkYQI77DqD9n8ELmEJWJNxw8fJVytTLA4/7QAbGrdbZDm0XTjun5S84AEBo6NB1Fadl6CmwcjFfvrNhf2QrMHg766cuDHKnjupICpkYoDc8tRw0jH2/6yfH+cEAiO1fAEsx/N5QijFm8Nme/y8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1731920663; c=relaxed/simple; bh=4BQxpIeuu0nVwUe5p6hnFcoQh6vfzVx7Oksv1WzT3M0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=WuV9gtyWog7qi3qcY/GOuJkhwkZXXt+h2fzvnU8fxct5wV3tut2j6mOOD4ZkKfMlUYeMmwZeViWXbjmhJVAkBcBDTN5RXaRBNpjUUE2puNPuB3v+LoV701IguQGQEJzvdzNCvxZnzdOwNC79uKZGr0TzxnOHD9A//vXDM/Awf9c= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=suse.com; spf=pass smtp.mailfrom=suse.com; dkim=pass (2048-bit key) header.d=suse.com header.i=@suse.com header.b=KwJYT+1f; arc=none smtp.client-ip=209.85.218.47 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=suse.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=suse.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=suse.com header.i=@suse.com header.b="KwJYT+1f" Received: by mail-ej1-f47.google.com with SMTP id a640c23a62f3a-a9ed7d8d4e0so284610166b.1 for ; Mon, 18 Nov 2024 01:04:21 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=google; t=1731920660; x=1732525460; darn=vger.kernel.org; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=Ws/rbfiwFMNWy8k0tnCWfMeaJzWW6S4EceX0n9OoukE=; b=KwJYT+1fceF8Jn5+OtcCy8fP3MngMd6XLXKAYHXjgu7Rls5dmEQZJ9PwT/bKA7nf39 iYCoN2dMOp4MaFyscgrSRHZz48jxgaJGcXwAdg6NBVN2XaapOezeTgdTqd9YKHG7w78k 7q/p7O63+EHoSI2EUkuScs1Ww9KBc3oJofEpNCvwvZTTxK79psq3zUmb1F4h8eMCuwJ5 roLFKudouMPSmkbhVR6EhyV5xOkV92oyQaY3E6oBsU0lhaxvUyYziyEg6N8Bd4FMzpDp G2JBh1y3OR+l4Y3OwEHNsAkKGNSyoIYOQVqTWWjI4pIynO3OpluSTTF73Mrz0tFvsrg+ 7Lcw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1731920660; x=1732525460; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=Ws/rbfiwFMNWy8k0tnCWfMeaJzWW6S4EceX0n9OoukE=; b=asVoboz04OohqNSx0FhirHINv4xeyZ8dPWwL36ooi9/LJ6qiyqd1XfnoZNmQDg0K2/ EEwHf44HBh6vRWrYD9Eaq9FDmHYQJn/gZ96u4PWCqBaGbW6o+c2TixQ4fqT73vr2/W35 cRe78fhfbUNhNtuNNk8ZT0cGO8rneqaWVRQ0WeMwks0N9l7yvxCNJdwYT+hfLrztC5nT kPQS2rT1LlgEF07TFxOv1W4OosUoL0vr3fBe+IlH8Jh7/FU0BS/UnJHh6vl8Q9H6Lkqs laxfFDiYRPKEs76mGK8ay+iTEbiOP08FE94FVl9xU0RZ84S0oYdaQw/88ZmxkVVinfHp sHXA== X-Forwarded-Encrypted: i=1; AJvYcCUNS29Ya0x9HKFCXcIKq8yCFuNijNKyupUuw10RAMciTbJtptcLQalSp+tGJC8Tps2xAWnZIjknpgbts9Q=@vger.kernel.org X-Gm-Message-State: AOJu0YxWZMuaKkETsFeARGzpJnLzEDgNzXJTenmJ2NRDoswC2CYGI2r0 EqNeNKJKTIzt8AU/GZcM63xXSDzBu2Ke/xSQDil2X/i/lLEeoeUIEsYIZSJ1iSc= X-Google-Smtp-Source: AGHT+IHiDm/bz3WEh1enyRl0N1th54dk8RHN4GA3K1qzcDnriUz2+DDQ+dD96pBdIfYZ9dcNHYkGjA== X-Received: by 2002:a17:907:7284:b0:a9e:c263:29a7 with SMTP id a640c23a62f3a-aa483420a9amr1096106266b.23.1731920659889; Mon, 18 Nov 2024 01:04:19 -0800 (PST) Received: from localhost (109-81-88-120.rct.o2.cz. [109.81.88.120]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-aa20df50defsm513526566b.50.2024.11.18.01.04.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 18 Nov 2024 01:04:19 -0800 (PST) Date: Mon, 18 Nov 2024 10:04:18 +0100 From: Michal Hocko To: "Nysal Jan K.A." Cc: Andrew Morton , Michael Ellerman , Mathieu Desnoyers , Segher Boessenkool , Stephen Rothwell , Peter Zijlstra , linuxppc-dev@lists.ozlabs.org, Nathan Chancellor , Nick Desaulniers , Bill Wendling , Justin Stitt , Vlastimil Babka , "Liam R. Howlett" , Mark Brown , Kent Overstreet , Rick Edgecombe , linux-kernel@vger.kernel.org, llvm@lists.linux.dev Subject: Re: [PATCH v2] sched/membarrier: Fix redundant load of membarrier_state Message-ID: References: <20241007053936.833392-1-nysal@linux.ibm.com> <20241029055133.121418-1-nysal@linux.ibm.com> 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=us-ascii Content-Disposition: inline In-Reply-To: <20241029055133.121418-1-nysal@linux.ibm.com> I do not see this patch staged in any tree (e.g. linux-next). Is this on its way to be merged? Thanks! On Tue 29-10-24 11:21:28, Nysal Jan K.A. wrote: > On architectures where ARCH_HAS_SYNC_CORE_BEFORE_USERMODE > is not selected, sync_core_before_usermode() is a no-op. > In membarrier_mm_sync_core_before_usermode() the compiler does not > eliminate redundant branches and load of mm->membarrier_state > for this case as the atomic_read() cannot be optimized away. > > Here's a snippet of the code generated for finish_task_switch() on powerpc > prior to this change: > > 1b786c: ld r26,2624(r30) # mm = rq->prev_mm; > ....... > 1b78c8: cmpdi cr7,r26,0 > 1b78cc: beq cr7,1b78e4 > 1b78d0: ld r9,2312(r13) # current > 1b78d4: ld r9,1888(r9) # current->mm > 1b78d8: cmpd cr7,r26,r9 > 1b78dc: beq cr7,1b7a70 > 1b78e0: hwsync > 1b78e4: cmplwi cr7,r27,128 > ....... > 1b7a70: lwz r9,176(r26) # atomic_read(&mm->membarrier_state) > 1b7a74: b 1b78e0 > > This was found while analyzing "perf c2c" reports on kernels prior > to commit c1753fd02a00 ("mm: move mm_count into its own cache line") > where mm_count was false sharing with membarrier_state. > > There is a minor improvement in the size of finish_task_switch(). > The following are results from bloat-o-meter for ppc64le: > > GCC 7.5.0 > --------- > add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-32 (-32) > Function old new delta > finish_task_switch 884 852 -32 > > GCC 12.2.1 > ---------- > add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-32 (-32) > Function old new delta > finish_task_switch.isra 852 820 -32 > > LLVM 17.0.6 > ----------- > add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-36 (-36) > Function old new delta > rt_mutex_schedule 120 104 -16 > finish_task_switch 792 772 -20 > > Results on aarch64: > > GCC 14.1.1 > ---------- > add/remove: 0/2 grow/shrink: 1/1 up/down: 4/-60 (-56) > Function old new delta > get_nohz_timer_target 352 356 +4 > e843419@0b02_0000d7e7_408 8 - -8 > e843419@01bb_000021d2_868 8 - -8 > finish_task_switch.isra 592 548 -44 > > Signed-off-by: Nysal Jan K.A. > --- > V1 -> V2: > - Add results for aarch64 > - Add a comment describing the changes > --- > include/linux/sched/mm.h | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h > index 928a626725e6..b13474825130 100644 > --- a/include/linux/sched/mm.h > +++ b/include/linux/sched/mm.h > @@ -531,6 +531,13 @@ enum { > > static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm) > { > + /* > + * The atomic_read() below prevents CSE. The following should > + * help the compiler generate more efficient code on architectures > + * where sync_core_before_usermode() is a no-op. > + */ > + if (!IS_ENABLED(CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE)) > + return; > if (current->mm != mm) > return; > if (likely(!(atomic_read(&mm->membarrier_state) & > -- > 2.47.0 > -- Michal Hocko SUSE Labs