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 69929CDB474 for ; Mon, 23 Oct 2023 08:32:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232494AbjJWIc3 (ORCPT ); Mon, 23 Oct 2023 04:32:29 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55414 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229609AbjJWIcD (ORCPT ); Mon, 23 Oct 2023 04:32:03 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 17678D6B for ; Mon, 23 Oct 2023 01:31:04 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1698049863; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=HOnHu/HXv+/MexAzD91ndTElaP4qcEAtusLsQWzhRN8=; b=DNQUBabz36CAvYMTBzJAUI9AoGWrV13MsJUfsgo5Pv9MznbA9wvVwSuYaEeDUeqasdLHkB su4c4yIMQ0moO0vYFKIydGEshtBXoFxLIEWvNmRoJOzUyYRCja8DtRuU5zhXSDGFrcTJmR oPGPC/TMQU1Xn+/eSofJhSEKQm8N+qk= Received: from mail-wr1-f72.google.com (mail-wr1-f72.google.com [209.85.221.72]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-399-6Zn4pOKYNLaIw1ZH9hr5oQ-1; Mon, 23 Oct 2023 04:31:01 -0400 X-MC-Unique: 6Zn4pOKYNLaIw1ZH9hr5oQ-1 Received: by mail-wr1-f72.google.com with SMTP id ffacd0b85a97d-32d879cac50so1388872f8f.0 for ; Mon, 23 Oct 2023 01:31:01 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1698049860; x=1698654660; h=content-transfer-encoding:in-reply-to:from:content-language :references:cc:to:subject:user-agent:mime-version:date:message-id :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=HOnHu/HXv+/MexAzD91ndTElaP4qcEAtusLsQWzhRN8=; b=paJLiGtkdqatx+2JXGn5sxw6v+8VM1WRd9zdP7iYzbd8Dfhn1cdb9GPltLPQ7fF82j zC03LGl5Vx73yl+M2LWcx0bdtHXrCgbe0arnD1zCSL8Ua3XARAdqFV5/PAv1XoRlNxZu N8PoNsoBMfrMKsJdq85WjdUUMtKi4UGSsOucPRJMG3emGcCRoEzepdNErKckMvz/yH6Z Yu3gkvBCjvAF802HmVoH9CusPC/rFUchD9dd2RyLE7clvtwAOlkdThBP+G2ihb9Hm+mD Ho5NSWpCMaSk7jXwIK+C3npesY9lC+oA0yVpIXfrFb1Tn12Lb4htAbMW0IzSwZBNVDpb xwYw== X-Gm-Message-State: AOJu0Yy5oBT0iDI5q2vXGSvPgrJ6vycdDdeDbZd+88dKai+HGJvJ+fSu tpHPAgsDl7XnwzQtCeEmw9g3+Pk/katGFPPh5WSBCUd+2TJ7w1mIXG/NWftSyrcqULGLL5iOypz e7bB32m3P5PQVsqGimwXtO1Qn X-Received: by 2002:adf:ecc8:0:b0:31f:fa61:961d with SMTP id s8-20020adfecc8000000b0031ffa61961dmr5420239wro.63.1698049860239; Mon, 23 Oct 2023 01:31:00 -0700 (PDT) X-Google-Smtp-Source: AGHT+IEsM0QtK3+WXTL5mHWBFb49plfkG+D7naag8X+Y3CsdzRTvOrT+44se+2kfRdM5vRG4iQ0u0g== X-Received: by 2002:adf:ecc8:0:b0:31f:fa61:961d with SMTP id s8-20020adfecc8000000b0031ffa61961dmr5420223wro.63.1698049859823; Mon, 23 Oct 2023 01:30:59 -0700 (PDT) Received: from ?IPV6:2a01:e0a:d5:a000:680e:9bf4:b6a9:959b? ([2a01:e0a:d5:a000:680e:9bf4:b6a9:959b]) by smtp.gmail.com with ESMTPSA id f7-20020adff587000000b0031fb91f23e9sm7233075wro.43.2023.10.23.01.30.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 23 Oct 2023 01:30:59 -0700 (PDT) Message-ID: <74b367bd-ac80-478b-8f82-e98cb6e40475@redhat.com> Date: Mon, 23 Oct 2023 10:30:58 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] drm/mgag200: Flush the cache to improve latency To: Thomas Zimmermann , dri-devel@lists.freedesktop.org, airlied@redhat.com, daniel@ffwll.ch Cc: Linux Kernel Mailing List References: <20231019135655.313759-1-jfalempe@redhat.com> <660c0260-0e22-4e9c-ab13-157adaa0b14d@suse.de> Content-Language: en-US From: Jocelyn Falempe In-Reply-To: <660c0260-0e22-4e9c-ab13-157adaa0b14d@suse.de> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 20/10/2023 14:06, Thomas Zimmermann wrote: > (cc'ing lkml for feedback) > > Hi Jocelyn > > Am 19.10.23 um 15:55 schrieb Jocelyn Falempe: >> We found a regression in v5.10 on real-time server, using the >> rt-kernel and the mgag200 driver. It's some really specialized >> workload, with <10us latency expectation on isolated core. >> After the v5.10, the real time tasks missed their <10us latency >> when something prints on the screen (fbcon or printk) > > I'd like to hear the opinion of the RT-devs on this patch. Because AFAIK > we never did such a workaround in other drivers. And AFAIK printk is a > PITA anyway. Most other drivers uses DMA, which means this workaround can't apply to them. > > IMHO if that RT system cannot handle differences in framebuffer caching, > it's under-powered. It's just a matter of time until something else > changes and the problem returns. And (honest question) as it's an > x86-64, how do they handle System Management Mode? I think it's not a big news, that the Matrox G200 from 1999 is under-powered. I was also a bit surprised that flushing the cache would have such effect on latency. The tests we are doing can run 24h with the workaround, without any interrupt taking more than 10us. Without the workaround, every ~30s the interrupt failed its 10us target. > >> >> The regression has been bisected to 2 commits: >> 0b34d58b6c32 ("drm/mgag200: Enable caching for SHMEM pages") >> 4862ffaec523 ("drm/mgag200: Move vmap out of commit tail") >> >> The first one changed the system memory framebuffer from Write-Combine >> to the default caching. >> Before the second commit, the mgag200 driver used to unmap the >> framebuffer after each frame, which implicitly does a cache flush. >> Both regressions are fixed by the following patch, which forces a >> cache flush after each frame, reverting to almost v5.9 behavior. > > With that second commit, we essentially never unmap an active > framebuffer console. But with commit > > 359c6649cd9a ("drm/gem: Implement shadow-plane {begin, end}_fb_access > with vmap") > > we now again unmap the console framebuffer after the pageflip happened. > > So how does the latest kernel behave wrt to the problem? The regression was found when upgrading the server from v5.4 to v5.14, so we didn't test with later kernels. We will test with v6.3 (which should have 359c6649cd9a ) and see what it gives. > >> This is necessary only if you have strong realtime constraints, so I >> put the cache flush under the CONFIG_PREEMPT_RT config flag. >> Also clflush is only availabe on x86, (and this issue has only been >> reproduced on x86_64) so it's also under the CONFIG_X86 config flag. >> >> Fixes: 0b34d58b6c32 ("drm/mgag200: Enable caching for SHMEM pages") >> Fixes: 4862ffaec523 ("drm/mgag200: Move vmap out of commit tail") >> Signed-off-by: Jocelyn Falempe >> --- >>   drivers/gpu/drm/mgag200/mgag200_mode.c | 5 +++++ >>   1 file changed, 5 insertions(+) >> >> diff --git a/drivers/gpu/drm/mgag200/mgag200_mode.c >> b/drivers/gpu/drm/mgag200/mgag200_mode.c >> index af3ce5a6a636..11660cd29cea 100644 >> --- a/drivers/gpu/drm/mgag200/mgag200_mode.c >> +++ b/drivers/gpu/drm/mgag200/mgag200_mode.c >> @@ -13,6 +13,7 @@ >>   #include >>   #include >> +#include >>   #include >>   #include >>   #include >> @@ -436,6 +437,10 @@ static void mgag200_handle_damage(struct >> mga_device *mdev, const struct iosys_ma >>       iosys_map_incr(&dst, drm_fb_clip_offset(fb->pitches[0], >> fb->format, clip)); >>       drm_fb_memcpy(&dst, fb->pitches, vmap, fb, clip); >> +    /* On RT systems, flushing the cache reduces the latency for >> other RT tasks */ >> +#if defined(CONFIG_X86) && defined(CONFIG_PREEMPT_RT) >> +    drm_clflush_virt_range(vmap, fb->height * fb->pitches[0]); >> +#endif > > Your second commit is part of a larger patchset that updates several > drivers. They might all be affected. So if anything, the patch should go > here before the unmap call: > > https://elixir.bootlin.com/linux/v6.5/source/drivers/gpu/drm/drm_gem_atomic_helper.c#L377 > The regression was found only with G200 currently, so I don't want to apply it blindly on other drivers. Thanks for your help, Best regards, -- Jocelyn > with a much expanded comment. > > But I'd really like to hear other peoples' opinions on the matter. > > Best regards > Thomas > >>   } >>   /* >> >> base-commit: 2dde18cd1d8fac735875f2e4987f11817cc0bc2c >