From mboxrd@z Thu Jan 1 00:00:00 1970 From: Johannes Weiner Subject: Re: UDP rx packet loss in a cgroup with a memory limit Date: Thu, 13 Oct 2022 10:22:10 -0400 Message-ID: References: Mime-Version: 1.0 Content-Transfer-Encoding: base64 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=cmpxchg-org.20210112.gappssmtp.com; s=20210112; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:message-id:subject:cc:to:from:date:from:to :cc:subject:date:message-id:reply-to; bh=2NGQAGAfwfgj6bQQFri3e7UvpFHJBbG66RlLh3PT3p8=; b=SDhA7zbv62PBaHsvPGDAgC+u2bWJOE2OO2P9PQ8O8yhfC3+O+6zOHA0pKrIUoxR0jb rdkdaIOIcP0Zm/LwhM9aYOF2nNRGLOCXW6KDvAxTI8GGHfmY59JXXY4vLfnHaGMnBvYm KPJqMc1JMbsz+TlNcGzW9lsTFb86rQ8TKR4SJ8rtNt8IG9eK3Gx2mc7rf0D0IzPdj0qj q94Q3OnfgWsGiDhsBjJzu33I6+YFoFGIH5067KhLkF04G3vqGwwO3xG4oVW1zsY4yO2E yyw4sfe4cfH5bzvYjNCPsf8dOL1soi9uAiw9XtpJ0HfB21bV+G9RFgpzYGiK9R9YarDJ hmOg== Content-Disposition: inline In-Reply-To: List-ID: Content-Type: text/plain; charset="macroman" To: Shakeel Butt Cc: =?utf-8?Q?Gra=C5=BEvydas?= Ignotas , Wei Wang , Eric Dumazet , netdev , Michal Hocko , Roman Gushchin , Linux MM , Cgroups T24gV2VkLCBPY3QgMTIsIDIwMjIgYXQgMDk6MzY6MzRQTSAtMDcwMCwgU2hha2VlbCBCdXR0IHdy b3RlOgo+IE9uIFdlZCwgQXVnIDE3LCAyMDIyIGF0IDE6MTIgUE0gR3Jhxb52eWRhcyBJZ25vdGFz IDxub3Rhc2FzLVJlNUpRRWVRcWU4QXZ4dGl1TXd4M3dAcHVibGljLmdtYW5lLm9yZz4gd3JvdGU6 Cj4gPgo+ID4gT24gV2VkLCBBdWcgMTcsIDIwMjIgYXQgOToxNiBQTSBXZWkgV2FuZyA8d2Vpd2Fu LWhwSXFzRDRBS2xmUVQwZFpSK0FsZkFAcHVibGljLmdtYW5lLm9yZz4gd3JvdGU6Cj4gPiA+Cj4g PiA+IE9uIFdlZCwgQXVnIDE3LCAyMDIyIGF0IDEwOjM3IEFNIFNoYWtlZWwgQnV0dCA8c2hha2Vl bGItaHBJcXNENEFLbGZRVDBkWlIrQWxmQUBwdWJsaWMuZ21hbmUub3JnPiB3cm90ZToKPiA+ID4g Pgo+ID4gPiA+ICsgRXJpYyBhbmQgbmV0ZGV2Cj4gPiA+ID4KPiA+ID4gPiBPbiBXZWQsIEF1ZyAx NywgMjAyMiBhdCAxMDoxMyBBTSBKb2hhbm5lcyBXZWluZXIgPGhhbm5lcy1kcnVVZ3ZsMExDTkFm dWdScEM2dTZ3QHB1YmxpYy5nbWFuZS5vcmc+IHdyb3RlOgo+ID4gPiA+ID4KPiA+ID4gPiA+IFRo 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Thu, 13 Oct 2022 07:22:11 -0700 (PDT) Date: Thu, 13 Oct 2022 10:22:10 -0400 From: Johannes Weiner To: Shakeel Butt Cc: =?utf-8?Q?Gra=C5=BEvydas?= Ignotas , Wei Wang , Eric Dumazet , netdev , Michal Hocko , Roman Gushchin , Linux MM , Cgroups Subject: Re: UDP rx packet loss in a cgroup with a memory limit Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: ARC-Authentication-Results: i=1; imf20.hostedemail.com; dkim=pass header.d=cmpxchg-org.20210112.gappssmtp.com header.s=20210112 header.b=SDhA7zbv; spf=pass (imf20.hostedemail.com: domain of hannes@cmpxchg.org designates 209.85.219.41 as permitted sender) smtp.mailfrom=hannes@cmpxchg.org; dmarc=pass (policy=none) header.from=cmpxchg.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1665670933; a=rsa-sha256; cv=none; b=rIwHmk4InplltHIBpIYSYkpf6vkq8b1nFEOVJ9dXsEPku0r3gDUrQZUdaBIAw8QfQhqm6v 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smtp.mailfrom=hannes@cmpxchg.org; dmarc=pass (policy=none) header.from=cmpxchg.org X-Stat-Signature: ezgw31gouu15kxop8outyrw5omfw4wbs X-Rspamd-Queue-Id: C0CF01C0036 X-HE-Tag: 1665670932-701416 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Oct 12, 2022 at 09:36:34PM -0700, Shakeel Butt wrote: > On Wed, Aug 17, 2022 at 1:12 PM GraÅžvydas Ignotas wrote: > > > > On Wed, Aug 17, 2022 at 9:16 PM Wei Wang wrote: > > > > > > On Wed, Aug 17, 2022 at 10:37 AM Shakeel Butt wrote: > > > > > > > > + Eric and netdev > > > > > > > > On Wed, Aug 17, 2022 at 10:13 AM Johannes Weiner wrote: > > > > > > > > > > This is most likely a regression caused by this patch: > > > > > > > > > > commit 4b1327be9fe57443295ae86fe0fcf24a18469e9f > > > > > Author: Wei Wang > > > > > Date: Tue Aug 17 12:40:03 2021 -0700 > > > > > > > > > > net-memcg: pass in gfp_t mask to mem_cgroup_charge_skmem() > > > > > > > > > > Add gfp_t mask as an input parameter to mem_cgroup_charge_skmem(), > > > > > to give more control to the networking stack and enable it to change > > > > > memcg charging behavior. In the future, the networking stack may decide > > > > > to avoid oom-kills when fallbacks are more appropriate. > > > > > > > > > > One behavior change in mem_cgroup_charge_skmem() by this patch is to > > > > > avoid force charging by default and let the caller decide when and if > > > > > force charging is needed through the presence or absence of > > > > > __GFP_NOFAIL. > > > > > > > > > > Signed-off-by: Wei Wang > > > > > Reviewed-by: Shakeel Butt > > > > > Signed-off-by: David S. Miller > > > > > > > > > > We never used to fail these allocations. Cgroups don't have a > > > > > kswapd-style watermark reclaimer, so the network relied on > > > > > force-charging and leaving reclaim to allocations that can block. > > > > > Now it seems network packets could just fail indefinitely. > > > > > > > > > > The changelog is a bit terse given how drastic the behavior change > > > > > is. Wei, Shakeel, can you fill in why this was changed? Can we revert > > > > > this for the time being? > > > > > > > > Does reverting the patch fix the issue? However I don't think it will. > > > > > > > > Please note that we still have the force charging as before this > > > > patch. Previously when mem_cgroup_charge_skmem() force charges, it > > > > returns false and __sk_mem_raise_allocated takes suppress_allocation > > > > code path. Based on some heuristics, it may allow it or it may > > > > uncharge and return failure. > > > > > > The force charging logic in __sk_mem_raise_allocated only gets > > > considered on tx path for STREAM socket. So it probably does not take > > > effect on UDP path. And, that logic is NOT being altered in the above > > > patch. > > > So specifically for UDP receive path, what happens in > > > __sk_mem_raise_allocated() BEFORE the above patch is: > > > - mem_cgroup_charge_skmem() gets called: > > > - try_charge() with GFP_NOWAIT gets called and failed > > > - try_charge() with __GFP_NOFAIL > > > - return false > > > - goto suppress_allocation: > > > - mem_cgroup_uncharge_skmem() gets called > > > - return 0 (which means failure) > > > > > > AFTER the above patch, what happens in __sk_mem_raise_allocated() is: > > > - mem_cgroup_charge_skmem() gets called: > > > - try_charge() with GFP_NOWAIT gets called and failed > > > - return false > > > - goto suppress_allocation: > > > - We no longer calls mem_cgroup_uncharge_skmem() > > > - return 0 > > > > > > So I agree with Shakeel, that this change shouldn't alter the behavior > > > of the above call path in such a situation. > > > But do let us know if reverting this change has any effect on your test. > > > > The problem is still there (the kernel wasn't compiling after revert, > > had to adjust another seemingly unrelated callsite). It's hard to tell > > if it's better or worse since it happens so randomly. > > > > Hello everyone, we have a better understanding why the patch pointed > out by Johannes might have exposed this issue. See > https://lore.kernel.org/all/20221013041833.rhifxw4gqwk4ofi2@google.com/. Wow, that's super subtle! Nice sleuthing. > To summarize, the old code was depending on a subtle interaction of > force-charge and percpu charge caches which this patch removed. The > fix I am proposing is for the network stack to be explicit of its need > (i.e. use GFP_ATOMIC) instead of depending on a subtle behavior. That sounds good to me.