From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id BFAF6C433EF for ; Thu, 9 Dec 2021 23:54:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234660AbhLIX6F (ORCPT ); Thu, 9 Dec 2021 18:58:05 -0500 Received: from mga12.intel.com ([192.55.52.136]:30467 "EHLO mga12.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234640AbhLIX6E (ORCPT ); Thu, 9 Dec 2021 18:58:04 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1639094070; x=1670630070; h=subject:to:cc:references:from:message-id:date: mime-version:in-reply-to:content-transfer-encoding; bh=+MW3HkDaIUBsZSaEtafCmC2qKsJHc0+FFJudrN28aQQ=; b=Oo1v3vFAyvY75AR8Nebjflqs9856wwf44n6yhVrn4ca8QG6u7jNsROJc o5nlbKTKvxOPu4WQ/znLRlnB7j30s1ss98tXTtWCS9lSl5VnDaME3CRxV 1W2vjhd7m4nOBLDgUJjzvjLt7k4YTzkKdYIk0mHKhht8DUdi9A2NQA1pp mXP3HRxsY3w6gdYN7EDENStqXW4QRf40SX8jJltkZccbU3h48ptM0qvMP pVXDM4kwC0/IRhxRlcewC0CTKYcWNcXr3qNf+EyTx9KE761tOzjLsxRXn UeZpI+40D1521ZX9l29mMhqWO9qDdox4duEUan/CD1Vr9kLKbdhaRptHo w==; X-IronPort-AV: E=McAfee;i="6200,9189,10193"; a="218254649" X-IronPort-AV: E=Sophos;i="5.88,194,1635231600"; d="scan'208";a="218254649" Received: from fmsmga003.fm.intel.com ([10.253.24.29]) by fmsmga106.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 09 Dec 2021 15:54:14 -0800 X-IronPort-AV: E=Sophos;i="5.88,193,1635231600"; d="scan'208";a="601887499" Received: from pmbutler-mobl.amr.corp.intel.com (HELO [10.212.131.7]) ([10.212.131.7]) by fmsmga003-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 09 Dec 2021 15:54:13 -0800 Subject: Re: [PATCH] signal: Skip the altstack update when not needed To: "Chang S. Bae" , linux-kernel@vger.kernel.org Cc: tglx@linutronix.de, dave.hansen@linux.intel.com, ebiederm@xmission.com, oleg@redhat.com, bp@alien8.de, x86@kernel.org References: <20211209232434.3585-1-chang.seok.bae@intel.com> From: Dave Hansen Autocrypt: addr=dave.hansen@intel.com; keydata= xsFNBE6HMP0BEADIMA3XYkQfF3dwHlj58Yjsc4E5y5G67cfbt8dvaUq2fx1lR0K9h1bOI6fC oAiUXvGAOxPDsB/P6UEOISPpLl5IuYsSwAeZGkdQ5g6m1xq7AlDJQZddhr/1DC/nMVa/2BoY 2UnKuZuSBu7lgOE193+7Uks3416N2hTkyKUSNkduyoZ9F5twiBhxPJwPtn/wnch6n5RsoXsb ygOEDxLEsSk/7eyFycjE+btUtAWZtx+HseyaGfqkZK0Z9bT1lsaHecmB203xShwCPT49Blxz VOab8668QpaEOdLGhtvrVYVK7x4skyT3nGWcgDCl5/Vp3TWA4K+IofwvXzX2ON/Mj7aQwf5W iC+3nWC7q0uxKwwsddJ0Nu+dpA/UORQWa1NiAftEoSpk5+nUUi0WE+5DRm0H+TXKBWMGNCFn c6+EKg5zQaa8KqymHcOrSXNPmzJuXvDQ8uj2J8XuzCZfK4uy1+YdIr0yyEMI7mdh4KX50LO1 pmowEqDh7dLShTOif/7UtQYrzYq9cPnjU2ZW4qd5Qz2joSGTG9eCXLz5PRe5SqHxv6ljk8mb ApNuY7bOXO/A7T2j5RwXIlcmssqIjBcxsRRoIbpCwWWGjkYjzYCjgsNFL6rt4OL11OUF37wL QcTl7fbCGv53KfKPdYD5hcbguLKi/aCccJK18ZwNjFhqr4MliQARAQABzShEYXZpZCBDaHJp c3RvcGhlciBIYW5zZW4gPGRhdmVAc3I3MS5uZXQ+wsF7BBMBAgAlAhsDBgsJCAcDAgYVCAIJ CgsEFgIDAQIeAQIXgAUCTo3k0QIZAQAKCRBoNZUwcMmSsMO2D/421Xg8pimb9mPzM5N7khT0 2MCnaGssU1T59YPE25kYdx2HntwdO0JA27Wn9xx5zYijOe6B21ufrvsyv42auCO85+oFJWfE K2R/IpLle09GDx5tcEmMAHX6KSxpHmGuJmUPibHVbfep2aCh9lKaDqQR07gXXWK5/yU1Dx0r VVFRaHTasp9fZ9AmY4K9/BSA3VkQ8v3OrxNty3OdsrmTTzO91YszpdbjjEFZK53zXy6tUD2d e1i0kBBS6NLAAsqEtneplz88T/v7MpLmpY30N9gQU3QyRC50jJ7LU9RazMjUQY1WohVsR56d ORqFxS8ChhyJs7BI34vQusYHDTp6PnZHUppb9WIzjeWlC7Jc8lSBDlEWodmqQQgp5+6AfhTD kDv1a+W5+ncq+Uo63WHRiCPuyt4di4/0zo28RVcjtzlGBZtmz2EIC3vUfmoZbO/Gn6EKbYAn rzz3iU/JWV8DwQ+sZSGu0HmvYMt6t5SmqWQo/hyHtA7uF5Wxtu1lCgolSQw4t49ZuOyOnQi5 f8R3nE7lpVCSF1TT+h8kMvFPv3VG7KunyjHr3sEptYxQs4VRxqeirSuyBv1TyxT+LdTm6j4a mulOWf+YtFRAgIYyyN5YOepDEBv4LUM8Tz98lZiNMlFyRMNrsLV6Pv6SxhrMxbT6TNVS5D+6 UorTLotDZKp5+M7BTQRUY85qARAAsgMW71BIXRgxjYNCYQ3Xs8k3TfAvQRbHccky50h99TUY sqdULbsb3KhmY29raw1bgmyM0a4DGS1YKN7qazCDsdQlxIJp9t2YYdBKXVRzPCCsfWe1dK/q 66UVhRPP8EGZ4CmFYuPTxqGY+dGRInxCeap/xzbKdvmPm01Iw3YFjAE4PQ4hTMr/H76KoDbD cq62U50oKC83ca/PRRh2QqEqACvIH4BR7jueAZSPEDnzwxvVgzyeuhwqHY05QRK/wsKuhq7s UuYtmN92Fasbxbw2tbVLZfoidklikvZAmotg0dwcFTjSRGEg0Gr3p/xBzJWNavFZZ95Rj7Et db0lCt0HDSY5q4GMR+SrFbH+jzUY/ZqfGdZCBqo0cdPPp58krVgtIGR+ja2Mkva6ah94/oQN lnCOw3udS+Eb/aRcM6detZr7XOngvxsWolBrhwTQFT9D2NH6ryAuvKd6yyAFt3/e7r+HHtkU kOy27D7IpjngqP+b4EumELI/NxPgIqT69PQmo9IZaI/oRaKorYnDaZrMXViqDrFdD37XELwQ gmLoSm2VfbOYY7fap/AhPOgOYOSqg3/Nxcapv71yoBzRRxOc4FxmZ65mn+q3rEM27yRztBW9 AnCKIc66T2i92HqXCw6AgoBJRjBkI3QnEkPgohQkZdAb8o9WGVKpfmZKbYBo4pEAEQEAAcLB XwQYAQIACQUCVGPOagIbDAAKCRBoNZUwcMmSsJeCEACCh7P/aaOLKWQxcnw47p4phIVR6pVL e4IEdR7Jf7ZL00s3vKSNT+nRqdl1ugJx9Ymsp8kXKMk9GSfmZpuMQB9c6io1qZc6nW/3TtvK pNGz7KPPtaDzvKA4S5tfrWPnDr7n15AU5vsIZvgMjU42gkbemkjJwP0B1RkifIK60yQqAAlT YZ14P0dIPdIPIlfEPiAWcg5BtLQU4Wg3cNQdpWrCJ1E3m/RIlXy/2Y3YOVVohfSy+4kvvYU3 lXUdPb04UPw4VWwjcVZPg7cgR7Izion61bGHqVqURgSALt2yvHl7cr68NYoFkzbNsGsye9ft M9ozM23JSgMkRylPSXTeh5JIK9pz2+etco3AfLCKtaRVysjvpysukmWMTrx8QnI5Nn5MOlJj 1Ov4/50JY9pXzgIDVSrgy6LYSMc4vKZ3QfCY7ipLRORyalFDF3j5AGCMRENJjHPD6O7bl3Xo 4DzMID+8eucbXxKiNEbs21IqBZbbKdY1GkcEGTE7AnkA3Y6YB7I/j9mQ3hCgm5muJuhM/2Fr OPsw5tV/LmQ5GXH0JQ/TZXWygyRFyyI2FqNTx4WHqUn3yFj8rwTAU1tluRUYyeLy0ayUlKBH ybj0N71vWO936MqP6haFERzuPAIpxj2ezwu0xb1GjTk4ynna6h5GjnKgdfOWoRtoWndMZxbA z5cecg== Message-ID: <64bb0617-bbba-f54d-bbf7-cdd77fa530b6@intel.com> Date: Thu, 9 Dec 2021 15:54:11 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20211209232434.3585-1-chang.seok.bae@intel.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 12/9/21 3:24 PM, Chang S. Bae wrote: > New x86 FPU features require a large signal stack for their large states. > Instead of requiring a large stack for every process, make sure enough > altstack both at sys_sigaltstack() and when enabling the feature in each > process. This is jumping into the imperative voice when describing the background. It's rather jarring > The optional size check was added. It helps to reject a too-small altstack > when the large feature is enabled. Also, the architecture code examines > each thread's altstack large enough before enabling the feature. > > But threads can be racy without a lock. So, this enforcement mechanism > accompanies a lock to serialize altstack updates and the size check. > > On the signal return path, the altstack is restored via do_sigaltstack(). > In fact, the threads without altstack ensure it is disabled there. While no > altstack change is needed in this case, this call ends up obtaining the > lock. > > When multiple signal returns hit the lock at the same time, this > unnecessarily increases the lock contention. > > Add a new check to avoid this. Check if an altstack update is needed. If > not, skip the lock and the update. This may help sys_sigaltstack() in > general. So place it in the function. How about: == Background == Support for large, "dynamic" fpstates was recently merged. This included code to ensure that sigaltstacks are sufficiently sized for these large states. A new lock was added to remove races between enabling large features and setting up sigaltstacks. == Problem == The new lock (sigaltstack_lock()) is acquired in the sigreturn path before restoring the old sigaltstack. Unfortunately, contention on the new lock causes a measurable signal handling performance regression[link here]. However, the common case is that no *changes* are made to the sigaltstack state at sigreturn. == Solution == do_sigaltstack() acquires sigaltstack_lock() and is used for both sys_sigaltstack() and restoring the sigaltstack in sys_sigreturn(). Check for changes to the sigaltstack before taking the lock. If no changes were made, return before acquiring the lock. This removes lock contention from the common-case sigreturn path. > diff --git a/kernel/signal.c b/kernel/signal.c > index a629b11bf3e0..eeb634f954cd 100644 > --- a/kernel/signal.c > +++ b/kernel/signal.c > @@ -4185,6 +4185,11 @@ do_sigaltstack (const stack_t *ss, stack_t *oss, unsigned long sp, > ss_mode != 0)) > return -EINVAL; > > + if (t->sas_ss_sp == (unsigned long)ss_sp && > + t->sas_ss_size == ss_size && > + t->sas_ss_flags == ss_flags) > + return 0; This needs something like: /* * Return before taking any locks if no actual * sigaltstack changes were requested. */ > sigaltstack_lock(); > if (ss_mode == SS_DISABLE) { > ss_size = 0; >