From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pf1-f175.google.com (mail-pf1-f175.google.com [209.85.210.175]) (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 9A6AF8BEF; Tue, 13 Aug 2024 01:54:22 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.210.175 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1723514064; cv=none; b=uKoZcbN6hke0iym2s04MFGGkfxtSUP1xCOlA7PTsKpzL4I8j6UHBSafg7odMgkpE4bjJdUDGYlXViIK1PgsruJ2cX5A8i8Y0DxJGp8KX3+7gtiFr2cE/RQ9dgOO8zOEk01GjQalae2XXjgLfUvfEOUoi2cfnsBUxxxVGoUGXotM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1723514064; c=relaxed/simple; bh=xf2c9r28DjNV2oZOIAFC3eioU11GutQquZEDO4E5AE4=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Xo2rxzc62Wjx8hawahX5obyh4/BG6scg8y6YsrbgGSm9bEO4TTABHUHJ+cdYz5Jn0RIbwWdW5/sYu658wdVgp2T3QHyVIbnSqlDYBLs6eQ03bN0qYqtNQYfScX9UAVq6/W99h7VlECLxS2GtXSU9QkVH3VHDpC/7wD+AHZAD2VU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=fomichev.me; spf=pass smtp.mailfrom=gmail.com; arc=none smtp.client-ip=209.85.210.175 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=fomichev.me Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Received: by mail-pf1-f175.google.com with SMTP id d2e1a72fcca58-70d1d6369acso4025948b3a.0; Mon, 12 Aug 2024 18:54:22 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1723514062; x=1724118862; 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=IR7nxsJ/on7OZmeb7iHI9ejZE2dXQFCt6FEeyf1PESQ=; b=nFd4KqfN0pjIAWqfYd1xBxEyiN2R0/F4HfwbHiSY5FfvvWiib+jkRbdrMNrIpb22MM 8LA15Knnp3z7aGomqI+Z6qumjLMIURqSDCJ6QvK0uk67XrJUO5IEPxnFFijuPFgA6YsL 28SB7X/QMSXqySzVEWWzqGtkB3w6rNsoBcmDCvMuDnhy3jnumRNXQy8OKURLdgGAoaI5 l8e3D1nvGKXXNig2jlhDLwJujAiOB3tfZKDu1sLdqjIF7LtJS0CV9DkYhdnRvyNcmYl+ oatlrtg+kUQtvsk/jhZsYBuJOASpOeyx/2OIQhIwvz8vGdz2OSOFmmK3ETDuM1UjafYp x4hw== X-Forwarded-Encrypted: i=1; AJvYcCXvWawhFOf6maOMXQpbICm+q5OsczGWPviz4ib48FvHA0tcAzs8dTb7lrkXY9QQAzFr63cuPENZJIdETtM3mKgevBeUER2cX8DngMn5Zh97bH7jbMCf3tzu36yzk393JwJ9+jDDtIGeuXo+yvop9qESA+hSOUtYsjbtF95nXaGqTpVkDQOuBg== X-Gm-Message-State: AOJu0YyA6DFomEADiIfkM+d8rSyKddtOt6kx0+Uremo7cC4LNa5l7oBJ qJqF0+qvb3NZ7RDHb6HysY1W32dT5nUO+olZLlHCEGsBwvYjKNA= X-Google-Smtp-Source: AGHT+IFFsSTbbZeJK3+KDsL25G7biWTsuXdC31iTRrYyxom4HI+FPmn4OkMb0AGCbqMleehQ0TaICA== X-Received: by 2002:a05:6a20:12c6:b0:1c4:8694:6be8 with SMTP id adf61e73a8af0-1c8da19049dmr2160169637.3.1723514061689; Mon, 12 Aug 2024 18:54:21 -0700 (PDT) Received: from localhost ([2601:646:9e00:f56e:73b6:7410:eb24:cba4]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-710e5ab7af4sm4603819b3a.208.2024.08.12.18.54.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 12 Aug 2024 18:54:21 -0700 (PDT) Date: Mon, 12 Aug 2024 18:54:20 -0700 From: Stanislav Fomichev To: Martin Karsten Cc: netdev@vger.kernel.org, Joe Damato , amritha.nambiar@intel.com, sridhar.samudrala@intel.com, Alexander Lobakin , Alexander Viro , Breno Leitao , Christian Brauner , Daniel Borkmann , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Jan Kara , Jiri Pirko , Johannes Berg , Jonathan Corbet , "open list:DOCUMENTATION" , "open list:FILESYSTEMS (VFS and infrastructure)" , open list , Lorenzo Bianconi , Paolo Abeni , Sebastian Andrzej Siewior Subject: Re: [RFC net-next 0/5] Suspend IRQs during preferred busy poll Message-ID: References: <20240812125717.413108-1-jdamato@fastly.com> <2bb121dd-3dcd-4142-ab87-02ccf4afd469@uwaterloo.ca> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: On 08/12, Martin Karsten wrote: > On 2024-08-12 19:03, Stanislav Fomichev wrote: > > On 08/12, Martin Karsten wrote: > > > On 2024-08-12 16:19, Stanislav Fomichev wrote: > > > > On 08/12, Joe Damato wrote: > > > > > Greetings: > > > > > > > > > > Martin Karsten (CC'd) and I have been collaborating on some ideas about > > > > > ways of reducing tail latency when using epoll-based busy poll and we'd > > > > > love to get feedback from the list on the code in this series. This is > > > > > the idea I mentioned at netdev conf, for those who were there. Barring > > > > > any major issues, we hope to submit this officially shortly after RFC. > > > > > > > > > > The basic idea for suspending IRQs in this manner was described in an > > > > > earlier paper presented at Sigmetrics 2024 [1]. > > > > > > > > Let me explicitly call out the paper. Very nice analysis! > > > > > > Thank you! > > > > > > [snip] > > > > > > > > Here's how it is intended to work: > > > > > - An administrator sets the existing sysfs parameters for > > > > > defer_hard_irqs and gro_flush_timeout to enable IRQ deferral. > > > > > > > > > > - An administrator sets the new sysfs parameter irq_suspend_timeout > > > > > to a larger value than gro-timeout to enable IRQ suspension. > > > > > > > > Can you expand more on what's the problem with the existing gro_flush_timeout? > > > > Is it defer_hard_irqs_count? Or you want a separate timeout only for the > > > > perfer_busy_poll case(why?)? Because looking at the first two patches, > > > > you essentially replace all usages of gro_flush_timeout with a new variable > > > > and I don't see how it helps. > > > > > > gro-flush-timeout (in combination with defer-hard-irqs) is the default irq > > > deferral mechanism and as such, always active when configured. Its static > > > periodic softirq processing leads to a situation where: > > > > > > - A long gro-flush-timeout causes high latencies when load is sufficiently > > > below capacity, or > > > > > > - a short gro-flush-timeout causes overhead when softirq execution > > > asynchronously competes with application processing at high load. > > > > > > The shortcomings of this are documented (to some extent) by our experiments. > > > See defer20 working well at low load, but having problems at high load, > > > while defer200 having higher latency at low load. > > > > > > irq-suspend-timeout is only active when an application uses > > > prefer-busy-polling and in that case, produces a nice alternating pattern of > > > application processing and networking processing (similar to what we > > > describe in the paper). This then works well with both low and high load. > > > > So you only want it for the prefer-busy-pollingc case, makes sense. I was > > a bit confused by the difference between defer200 and suspend200, > > but now I see that defer200 does not enable busypoll. > > > > I'm assuming that if you enable busypool in defer200 case, the numbers > > should be similar to suspend200 (ignoring potentially affecting > > non-busypolling queues due to higher gro_flush_timeout). > > defer200 + napi busy poll is essentially what we labelled "busy" and it does > not perform as well, since it still suffers interference between application > and softirq processing. With all your patches applied? Why? Userspace not keeping up? > > > > Maybe expand more on what code paths are we trying to improve? Existing > > > > busy polling code is not super readable, so would be nice to simplify > > > > it a bit in the process (if possible) instead of adding one more tunable. > > > > > > There are essentially three possible loops for network processing: > > > > > > 1) hardirq -> softirq -> napi poll; this is the baseline functionality > > > > > > 2) timer -> softirq -> napi poll; this is deferred irq processing scheme > > > with the shortcomings described above > > > > > > 3) epoll -> busy-poll -> napi poll > > > > > > If a system is configured for 1), not much can be done, as it is difficult > > > to interject anything into this loop without adding state and side effects. > > > This is what we tried for the paper, but it ended up being a hack. > > > > > > If however the system is configured for irq deferral, Loops 2) and 3) > > > "wrestle" with each other for control. Injecting the larger > > > irq-suspend-timeout for 'timer' in Loop 2) essentially tilts this in favour > > > of Loop 3) and creates the nice pattern describe above. > > > > And you hit (2) when the epoll goes to sleep and/or when the userspace > > isn't fast enough to keep up with the timer, presumably? I wonder > > if need to use this opportunity and do proper API as Joe hints in the > > cover letter. Something over netlink to say "I'm gonna busy-poll on > > this queue / napi_id and with this timeout". And then we can essentially make > > gro_flush_timeout per queue (and avoid > > napi_resume_irqs/napi_suspend_irqs). Existing gro_flush_timeout feels > > too hacky already :-( > > If someone would implement the necessary changes to make these parameters > per-napi, this would improve things further, but note that the current > proposal gives strong performance across a range of workloads, which is > otherwise difficult to impossible to achieve. Let's see what other people have to say. But we tried to do a similar setup at Google recently and getting all these parameters right was not trivial. Joe's recent patch series to push some of these into epoll context are a step in the right direction. It would be nice to have more explicit interface to express busy poling preference for the users vs chasing a bunch of global tunables and fighting against softirq wakups. > Note that napi_suspend_irqs/napi_resume_irqs is needed even for the sake of > an individual queue or application to make sure that IRQ suspension is > enabled/disabled right away when the state of the system changes from busy > to idle and back. Can we not handle everything in napi_busy_loop? If we can mark some napi contexts as "explicitly polled by userspace with a larger defer timeout", we should be able to do better compared to current NAPI_F_PREFER_BUSY_POLL which is more like "this particular napi_poll call is user busy polling". > > > [snip] > > > > > > > > - suspendX: > > > > > - set defer_hard_irqs to 100 > > > > > - set gro_flush_timeout to X,000 > > > > > - set irq_suspend_timeout to 20,000,000 > > > > > - enable busy poll via the existing ioctl (busy_poll_usecs = 0, > > > > > busy_poll_budget = 64, prefer_busy_poll = true) > > > > > > > > What's the intention of `busy_poll_usecs = 0` here? Presumably we fallback > > > > to busy_poll sysctl value? > > > > > > Before this patch set, ep_poll only calls napi_busy_poll, if busy_poll > > > (sysctl) or busy_poll_usecs is nonzero. However, this might lead to > > > busy-polling even when the application does not actually need or want it. > > > Only one iteration through the busy loop is needed to make the new scheme > > > work. Additional napi busy polling over and above is optional. > > > > Ack, thanks, was trying to understand why not stay with > > busy_poll_usecs=64 for consistency. But I guess you were just > > trying to show that patch 4/5 works. > > Right, and we would potentially be wasting CPU cycles by adding more > busy-looping. Or potentially improving the latency more if you happen to get more packets during busy_poll_usecs duration? I'd imagine some applications might prefer to 100% busy poll without ever going to sleep (that would probably require getting rid of napi_id tracking in epoll, but that's a different story).