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 gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 3434EC4332F for ; Wed, 2 Nov 2022 08:17:46 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id E879310E0CC; Wed, 2 Nov 2022 08:17:38 +0000 (UTC) Received: from mga01.intel.com (mga01.intel.com [192.55.52.88]) by gabe.freedesktop.org (Postfix) with ESMTPS id 50FB110E07E; Wed, 2 Nov 2022 08:17:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1667377054; x=1698913054; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=dg1bQMy2r8GKZVY+GmS9dZBf9Mqhoetuv0x9jhOVx6k=; b=aPhpA/FePCoJFoiO1o7Vr5t76UOi7D59Dwo9EmUI1Ekt3pDCXGZhIlLy +5Luop7CPVVu3O/UMyeRTKqtAqV8bdHL4xWpC7ZnCrz3nNcIoTBptYdMs jvtA0D6qKKMoqDefdULFjiTWChoPn91AIK7O1pUbhtvOjM+KK8vn2Ks4l TdqU1gyZ0DqP6mLpDsrugMLyZwDXzg6HO74nAV5x/8eXLzGLmijijEfd2 6FVzWQS1Gf1or2xNNg3iI15VwYzAWhkl515azh9tzkNZoyJlJXeB1VWYy SkXs7c+mBO/67SiQ3NoZbPbTQz/4Xnoydy61iT2PjRTsmQx63PMN2is5I A==; X-IronPort-AV: E=McAfee;i="6500,9779,10518"; a="336021075" X-IronPort-AV: E=Sophos;i="5.95,232,1661842800"; d="scan'208";a="336021075" Received: from fmsmga001.fm.intel.com ([10.253.24.23]) by fmsmga101.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 02 Nov 2022 01:17:33 -0700 X-IronPort-AV: E=McAfee;i="6500,9779,10518"; a="776806034" X-IronPort-AV: E=Sophos;i="5.95,232,1661842800"; d="scan'208";a="776806034" Received: from dsloan-mobl.ger.corp.intel.com (HELO [10.213.207.205]) ([10.213.207.205]) by fmsmga001-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 02 Nov 2022 01:17:32 -0700 Message-ID: <8f9cb0df-d2aa-e35a-1e91-845219b315a0@linux.intel.com> Date: Wed, 2 Nov 2022 08:17:31 +0000 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.3.3 Content-Language: en-US To: John Harrison , Intel-GFX@Lists.FreeDesktop.Org References: <20221028194649.1130223-1-John.C.Harrison@Intel.com> <20221028194649.1130223-3-John.C.Harrison@Intel.com> <56a6c98a-069c-77b0-d6c5-4575bc324075@linux.intel.com> <2e7cf7c4-76cb-5a69-8a61-7d1da3577060@linux.intel.com> <3f83f31f-28d4-5b68-3066-5a0b58e20e56@intel.com> <95404887-f886-078f-c644-635ada7d8bee@linux.intel.com> <4eae646a-345b-40d7-1ac6-4de027ee1fd4@intel.com> From: Tvrtko Ursulin Organization: Intel Corporation UK Plc In-Reply-To: <4eae646a-345b-40d7-1ac6-4de027ee1fd4@intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Subject: Re: [Intel-gfx] [PATCH 2/2] drm/i915/guc: Don't deadlock busyness stats vs reset X-BeenThere: intel-gfx@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Intel graphics driver community testing & development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: DRI-Devel@Lists.FreeDesktop.Org Errors-To: intel-gfx-bounces@lists.freedesktop.org Sender: "Intel-gfx" On 01/11/2022 16:56, John Harrison wrote: > On 11/1/2022 02:58, Tvrtko Ursulin wrote: >> On 31/10/2022 18:30, John Harrison wrote: >>> On 10/31/2022 05:51, Tvrtko Ursulin wrote: >>>> On 31/10/2022 10:09, Tvrtko Ursulin wrote: >>>>> On 28/10/2022 20:46, John.C.Harrison@Intel.com wrote: >>>>>> From: John Harrison >>>>>> >>>>>> The engine busyness stats has a worker function to do things like >>>>>> 64bit extend the 32bit hardware counters. The GuC's reset prepare >>>>>> function flushes out this worker function to ensure no corruption >>>>>> happens during the reset. Unforunately, the worker function has an >>>>>> infinite wait for active resets to finish before doing its work. Thus >>>>>> a deadlock would occur if the worker function had actually started >>>>>> just as the reset starts. >>>>>> >>>>>> Update the worker to abort if a reset is in progress rather than >>>>>> waiting for it to complete. It will still acquire the reset lock in >>>>>> the case where a reset was not already in progress. So the processing >>>>>> is still safe from corruption, but the deadlock can no longer occur. >>>>>> >>>>>> Signed-off-by: John Harrison >>>>>> --- >>>>>>   drivers/gpu/drm/i915/gt/intel_reset.c             | 15 >>>>>> ++++++++++++++- >>>>>>   drivers/gpu/drm/i915/gt/intel_reset.h             |  1 + >>>>>>   drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c |  6 ++++-- >>>>>>   3 files changed, 19 insertions(+), 3 deletions(-) >>>>>> >>>>>> diff --git a/drivers/gpu/drm/i915/gt/intel_reset.c >>>>>> b/drivers/gpu/drm/i915/gt/intel_reset.c >>>>>> index 3159df6cdd492..2f48c6e4420ea 100644 >>>>>> --- a/drivers/gpu/drm/i915/gt/intel_reset.c >>>>>> +++ b/drivers/gpu/drm/i915/gt/intel_reset.c >>>>>> @@ -1407,7 +1407,7 @@ void intel_gt_handle_error(struct intel_gt *gt, >>>>>>       intel_runtime_pm_put(gt->uncore->rpm, wakeref); >>>>>>   } >>>>>> -int intel_gt_reset_trylock(struct intel_gt *gt, int *srcu) >>>>>> +static int _intel_gt_reset_trylock(struct intel_gt *gt, int >>>>>> *srcu, bool retry) >>>>>>   { >>>>>>       might_lock(>->reset.backoff_srcu); >>>>>>       might_sleep(); >>>>>> @@ -1416,6 +1416,9 @@ int intel_gt_reset_trylock(struct intel_gt >>>>>> *gt, int *srcu) >>>>>>       while (test_bit(I915_RESET_BACKOFF, >->reset.flags)) { >>>>>>           rcu_read_unlock(); >>>>>> +        if (!retry) >>>>>> +            return -EBUSY; >>>>>> + >>>>>>           if (wait_event_interruptible(gt->reset.queue, >>>>>>                            !test_bit(I915_RESET_BACKOFF, >>>>>> >->reset.flags))) >>>>> >>>>> Would it be more obvious to rename the existing semantics to >>>>> intel_gt_reset_interruptible(), while the flavour you add in this >>>>> patch truly is trylock? I am not sure, since it's all a bit >>>>> special, but trylock sure feels confusing if it can sleep forever... >>> To me, it would seem totally more obvious to have a function called >>> 'trylock' not wait forever until it can manage to acquire the lock. >>> However, according to '2caffbf1176256 drm/i915: Revoke mmaps and >>> prevent access to fence registers across reset', the current >>> behaviour is exactly how the code was originally written and >>> intended. It hasn't just mutated into some confused evolution a >>> thousand patches later. So I figure there is some subtle but >>> important reason why it was named how it is named and yet does what >>> it does. Therefore it seemed safest to not change it unnecessarily. >> >> Yeah I looked at that but honestly I don't see the trylock semantics >> anywhere. The only failure to lock path comes from >> wait_event_interruptible. It could have easily been just a naming mishap. >> >> And I find adding a retry parameter to something called trylock makes >> this even more non-intuitive and would personally rather rename it >> all. Proof in the pudding is that the trylock naming did bite during >> development and review of the code this patch is now fixing. >> >> I do however understand your point about a degree of uncertainty but >> my feeling is to rather err on the side of obvious naming. Shall we >> ask for a third opinion? > Umesh had commented (internally) that the naming seems wrong and would > be good to change it. So we already have a third :). > > To be clear, you are thinking to keep the wrappers but rename to > intel_gt_reset_trylock() [retry = false] and > intel_gt_reset_interruptible() [retry = true]? Which will obviously > involve updating all but one existing user to use the interruptible name > as the existing name will change behaviour in a backwards breaking manner. Yes, intel_gt_reset_lock_interruptible and intel_gt_reset_trylock. I don't get the behaviour breaking part? Only the name will change. And amount of churn does not seem a problem: $ grep intel_gt_reset_trylock -r . ./gem/i915_gem_mman.c: ret = intel_gt_reset_trylock(ggtt->vm.gt, &srcu); ./gt/uc/intel_guc_submission.c: ret = intel_gt_reset_trylock(gt, &srcu); ./gt/intel_reset.c:int intel_gt_reset_trylock(struct intel_gt *gt, int *srcu) ./gt/intel_reset.h:int __must_check intel_gt_reset_trylock(struct intel_gt *gt, int *srcu) Regards, Tvrtko > > John. > >> >>>> Oh and might_sleep() shouldn't be there with the trylock version - I >>>> mean any flavour of the real trylock. >>> You mean if the code is split into two completely separate functions? >>> Or do you just mean to wrap the might_sleep() call with 'if(!retry)'? >>> >>> And just to be totally clear, the unconditional call to >>> rcu_read_lock() is not something that can sleep? One doesn't need a >>> might_sleep() before doing that lock? >> >> Corrrect, rcu_read_lock() can not sleep - it just disables preemption. >> So leaving the unconditional might_sleep() would have opportunity for >> false positives. >> >> Regards, >> >> Tvrtko >