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 X-Spam-Level: X-Spam-Status: No, score=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 21C67C43331 for ; Thu, 26 Mar 2020 18:02:57 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id F33A320714 for ; Thu, 26 Mar 2020 18:02:56 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org F33A320714 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=intel.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=intel-gfx-bounces@lists.freedesktop.org Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 75F116E20F; Thu, 26 Mar 2020 18:02:56 +0000 (UTC) Received: from mga18.intel.com (mga18.intel.com [134.134.136.126]) by gabe.freedesktop.org (Postfix) with ESMTPS id 791B56E20F for ; Thu, 26 Mar 2020 18:02:54 +0000 (UTC) IronPort-SDR: H8w6WhK8NLA0ijT8M/BM3Wl4eAN6DFmAplrc9+07ai4hkuYoxhdE3VdngrNi4aBNSqjbsPdlDv cFblwy6z33tw== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from fmsmga008.fm.intel.com ([10.253.24.58]) by orsmga106.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 26 Mar 2020 11:02:48 -0700 IronPort-SDR: uBnlBdISTueBGMfM7sSDUNcJyvGfDEJRVxAb2Qc4+C6S9pYtDh1lj0uhwWeb4Ysz2SU+P/JfAe X4vZkkTDCUyw== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.72,309,1580803200"; d="scan'208";a="240744071" Received: from orsosgc001.ra.intel.com (HELO orsosgc001.amr.corp.intel.com) ([10.23.184.150]) by fmsmga008.fm.intel.com with ESMTP; 26 Mar 2020 11:02:46 -0700 Date: Thu, 26 Mar 2020 11:02:46 -0700 From: Umesh Nerlige Ramappa To: "Dixit, Ashutosh" Message-ID: <20200326180246.GB9553@orsosgc001.amr.corp.intel.com> References: <98393a3bbd2652886d895e1c3250e43c6e0f1a24.1585160174.git.ashutosh.dixit@intel.com> <20200326003235.GA9553@orsosgc001.amr.corp.intel.com> <877dz7ubkb.wl-ashutosh.dixit@intel.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <877dz7ubkb.wl-ashutosh.dixit@intel.com> User-Agent: Mutt/1.12.0 (2019-05-25) Subject: Re: [Intel-gfx] [PATCH] drm/i915/perf: Do not clear pollin for small user read buffers 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: intel-gfx@lists.freedesktop.org Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Errors-To: intel-gfx-bounces@lists.freedesktop.org Sender: "Intel-gfx" On Wed, Mar 25, 2020 at 06:52:52PM -0700, Dixit, Ashutosh wrote: >On Wed, 25 Mar 2020 17:32:35 -0700, Umesh Nerlige Ramappa wrote: >> >> On Wed, Mar 25, 2020 at 11:20:19AM -0700, Ashutosh Dixit wrote: >> > It is wrong to block the user thread in the next poll when OA data is >> > already available which could not fit in the user buffer provided in >> > the previous read. In several cases the exact user buffer size is not >> > known. Blocking user space in poll can lead to data loss when the >> > buffer size used is smaller than the available data. >> > >> > This change fixes this issue and allows user space to read all OA data >> > even when using a buffer size smaller than the available data using >> > multiple non-blocking reads rather than staying blocked in poll till >> > the next timer interrupt. >> > >> > Cc: Umesh Nerlige Ramappa >> > Cc: Lionel Landwerlin >> > Signed-off-by: Ashutosh Dixit >> > --- >> > drivers/gpu/drm/i915/i915_perf.c | 62 ++++++-------------------------- >> > 1 file changed, 11 insertions(+), 51 deletions(-) >> > >> > diff --git a/drivers/gpu/drm/i915/i915_perf.c b/drivers/gpu/drm/i915/i915_perf.c >> > index 3222f6cd8255..c1a47c030941 100644 >> > --- a/drivers/gpu/drm/i915/i915_perf.c >> > +++ b/drivers/gpu/drm/i915/i915_perf.c >> > @@ -2957,49 +2957,6 @@ void i915_oa_init_reg_state(const struct intel_context *ce, >> > gen8_update_reg_state_unlocked(ce, stream); >> > } >> > >> > -/** >> > - * i915_perf_read_locked - &i915_perf_stream_ops->read with error normalisation >> > - * @stream: An i915 perf stream >> > - * @file: An i915 perf stream file >> > - * @buf: destination buffer given by userspace >> > - * @count: the number of bytes userspace wants to read >> > - * @ppos: (inout) file seek position (unused) >> > - * >> > - * Besides wrapping &i915_perf_stream_ops->read this provides a common place to >> > - * ensure that if we've successfully copied any data then reporting that takes >> > - * precedence over any internal error status, so the data isn't lost. >> > - * >> > - * For example ret will be -ENOSPC whenever there is more buffered data than >> > - * can be copied to userspace, but that's only interesting if we weren't able >> > - * to copy some data because it implies the userspace buffer is too small to >> > - * receive a single record (and we never split records). >> > - * >> > - * Another case with ret == -EFAULT is more of a grey area since it would seem >> > - * like bad form for userspace to ask us to overrun its buffer, but the user >> > - * knows best: >> > - * >> > - * http://yarchive.net/comp/linux/partial_reads_writes.html >> > - * >> > - * Returns: The number of bytes copied or a negative error code on failure. >> > - */ >> > -static ssize_t i915_perf_read_locked(struct i915_perf_stream *stream, >> > - struct file *file, >> > - char __user *buf, >> > - size_t count, >> > - loff_t *ppos) >> > -{ >> > - /* Note we keep the offset (aka bytes read) separate from any >> > - * error status so that the final check for whether we return >> > - * the bytes read with a higher precedence than any error (see >> > - * comment below) doesn't need to be handled/duplicated in >> > - * stream->ops->read() implementations. >> > - */ >> > - size_t offset = 0; >> > - int ret = stream->ops->read(stream, buf, count, &offset); >> > - >> > - return offset ?: (ret ?: -EAGAIN); >> > -} >> > - >> > /** >> > * i915_perf_read - handles read() FOP for i915 perf stream FDs >> > * @file: An i915 perf stream file >> > @@ -3025,6 +2982,8 @@ static ssize_t i915_perf_read(struct file *file, >> > { >> > struct i915_perf_stream *stream = file->private_data; >> > struct i915_perf *perf = stream->perf; >> > + size_t offset = 0; >> > + int __ret; >> > ssize_t ret; >> > >> > /* To ensure it's handled consistently we simply treat all reads of a >> > @@ -3048,16 +3007,18 @@ static ssize_t i915_perf_read(struct file *file, >> > return ret; >> > >> > mutex_lock(&perf->lock); >> > - ret = i915_perf_read_locked(stream, file, >> > - buf, count, ppos); >> > + __ret = stream->ops->read(stream, buf, count, &offset); >> > mutex_unlock(&perf->lock); >> > } while (ret == -EAGAIN); >> >> ret will never be EAGAIN here in the while. EAGAIN was returned by the >> deleted function in this patch if offset and ret are both 0. > >Good catch, I was so focussed on the non-blocking case that I missed the >blocking case. > >> Although I don't see how that would be true. > >As you say above, the old function i915_perf_read_locked() was doing this: > > return offset ?: (__ret ?: -EAGAIN); > >So -EAGAIN is returned from i915_perf_read_locked() when there is no data >to read but otherwise there is no other error. Since this is blocking read >we cannot return -EAGAIN to user space (since there is no data to read), we >must go back and block again. That is the purpose of the while loop. I >broke this logic in this patch and will need to fix this. > >> >> > } else { >> > mutex_lock(&perf->lock); >> > - ret = i915_perf_read_locked(stream, file, buf, count, ppos); >> > + __ret = stream->ops->read(stream, buf, count, &offset); >> > mutex_unlock(&perf->lock); >> > } >> > >> > + /* Possible values for __ret are 0, -EFAULT, -ENOSPC, -EAGAIN, ... */ >> >> __ret may never be EAGAIN either (comment^). I don't see EAGAIN in the read >> path. > >It's here: > >gen8_append_oa_reports() >{ > > /* > * An invalid tail pointer here means we're still waiting for the poll > * hrtimer callback to give us a pointer > */ > if (tail == INVALID_TAIL_PTR) > return -EAGAIN; >} Oh, you are right, EAGAIN is returned here. I was looking for it with the poll period patch series applied and these references are removed in that series. Thanks, Umesh > >> That said, EAGAIN seems to have been introduced in the prior code >> specifically for retrying the blocking read and may not have much meaning >> otherwise. > >No that's not true. The kernel non-blocking read() function (in fops) >returns -EAGAIN when there is no data to read (the function never returns 0 >except in case of EOF, in i915 perf code there is no EOF so read never >returns 0). This logic is the same as that in the previous code and we need >to preserve it. > >Will post a v2 with the fix. > >Thanks! >-- >Ashutosh > > >> >> Thanks, >> Umesh >> >> > + ret = offset ?: (__ret ?: -EAGAIN); >> > + >> > /* We allow the poll checking to sometimes report false positive EPOLLIN >> > * events where we might actually report EAGAIN on read() if there's >> > * not really any data available. In this situation though we don't >> > @@ -3065,13 +3026,12 @@ static ssize_t i915_perf_read(struct file *file, >> > * and read() returning -EAGAIN. Clearing the oa.pollin state here >> > * effectively ensures we back off until the next hrtimer callback >> > * before reporting another EPOLLIN event. >> > + * The exception to this is if ops->read() returned -ENOSPC which means >> > + * that more OA data is available than could fit in the user provided >> > + * buffer. In this case we want the next poll() call to not block. >> > */ >> > - if (ret >= 0 || ret == -EAGAIN) { >> > - /* Maybe make ->pollin per-stream state if we support multiple >> > - * concurrent streams in the future. >> > - */ >> > + if ((ret > 0 || ret == -EAGAIN) && __ret != -ENOSPC) >> > stream->pollin = false; >> > - } >> > >> > return ret; >> > } >> > -- >> > 2.25.2 >> > _______________________________________________ Intel-gfx mailing list Intel-gfx@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/intel-gfx