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 lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (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 26543CCD1AB for ; Fri, 24 Oct 2025 05:17:01 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [127.0.0.1]) by lists.ozlabs.org (Postfix) with ESMTP id 4ctB3H2gFnz301K; Fri, 24 Oct 2025 16:16:59 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; arc=none smtp.remote-ip="2607:f8b0:4864:20::1049" ARC-Seal: i=1; a=rsa-sha256; d=lists.ozlabs.org; s=201707; t=1761283019; cv=none; b=GzzVXydFRr3B11oKQL3aXiafcXfzquWR8/8vgp0gIOid6YHXRgeL06zG+9GAFKn/gUdG2k1Qew7y09VZoLeVERF2RCY+5DkR4q0x8FiPkAum86AcYa2ViDTk4AuG28jZ0PWE77yOGsTR/1nh+Cyu4+Lc9E+lWdyjNMpI8nqOYemFgvrIYjhloAkAHcHMecj76+RzmGAPq+FGIsUDRuLtAsNxhlPSCAsvKCyf8g/o658w3SeF5Hqen7f/0/SQSK6msIwbxiHgAQ9tx3MNFrZXwosLbkIdYzPNQbmpL8v05irrtYuJVSkxG0MZY4d6foWJ1Vr42Fnl3rfoXZMM2QrK5Q== ARC-Message-Signature: i=1; a=rsa-sha256; d=lists.ozlabs.org; s=201707; t=1761283019; c=relaxed/relaxed; bh=1JwAfOvfHWiveHwG3B+VPqzu+ErYo2MF7kqHuEps3qs=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=ElMTYxsAxo/KPtXP8wXz2gjB3vWGX5ZC1hDYimZv8jl+o589jEbSRdbqdeevBWARLmHofRnp+88QCpXZVOvm8vHt5mV8LuEw6yDgy7x9ObPwYmWDzIse9m2pCHyr7ikdrVZ4/QMmbqWBQh6FfhjCtQbuTcwm1ZbuBqyCYoYsroQFuB8NEYUusDrSONvKtOK+JRcmANjKPcZmCuCz9Wpmo/f1+nWJ9sSIyII2IsDUS09wyagkcfxFocesDkOhER/ufFxh2cR62MS4XAkA9IYARCY4+O0eocZPL6Gj4AcX+yPwva/CCJ5sdhLWRtQQwXzFvqd/USSpTzasPBKKSx4jgg== ARC-Authentication-Results: i=1; lists.ozlabs.org; dmarc=pass (p=reject dis=none) header.from=google.com; dkim=pass (2048-bit key; unprotected) header.d=google.com header.i=@google.com header.a=rsa-sha256 header.s=20230601 header.b=seaotke7; dkim-atps=neutral; spf=pass (client-ip=2607:f8b0:4864:20::1049; helo=mail-pj1-x1049.google.com; envelope-from=3xgv7aaykdbk9jc7nj5dd5a3.1dba7cjmee1-23ka7hih.doaz0h.dg5@flex--kuniyu.bounces.google.com; receiver=lists.ozlabs.org) smtp.mailfrom=flex--kuniyu.bounces.google.com Authentication-Results: lists.ozlabs.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: lists.ozlabs.org; dkim=pass (2048-bit key; unprotected) header.d=google.com header.i=@google.com header.a=rsa-sha256 header.s=20230601 header.b=seaotke7; dkim-atps=neutral Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=flex--kuniyu.bounces.google.com (client-ip=2607:f8b0:4864:20::1049; helo=mail-pj1-x1049.google.com; envelope-from=3xgv7aaykdbk9jc7nj5dd5a3.1dba7cjmee1-23ka7hih.doaz0h.dg5@flex--kuniyu.bounces.google.com; receiver=lists.ozlabs.org) Received: from mail-pj1-x1049.google.com (mail-pj1-x1049.google.com [IPv6:2607:f8b0:4864:20::1049]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4ctB3G0dSSz2yl2 for ; Fri, 24 Oct 2025 16:16:57 +1100 (AEDT) Received: by mail-pj1-x1049.google.com with SMTP id 98e67ed59e1d1-32ee62ed6beso2576271a91.2 for ; Thu, 23 Oct 2025 22:16:57 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1761283015; x=1761887815; darn=lists.ozlabs.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=1JwAfOvfHWiveHwG3B+VPqzu+ErYo2MF7kqHuEps3qs=; b=seaotke7hRnVFv+ltCSnRapIIKfW7viP8uuElGRVykhKKQJWB32WNVxoda2GB8lion aNQOzVrSz/FMkfzZGIRsO+hArh3YMnidAjQGfB1Ki8T4uR9rxkc0TQ2DSRx43Et59/p4 JLi84VjK1qRlGwkg+cTldYHmW9fQ8Naz9pqBH+B44xWnsJoHd/mY0ezPsfQ6fSWQB0CR DPDY/DqgAm7VSkhzqWkfPqx3bJmkowfQsyBYcDD9LOtqi+tO3rcUg69TStJR1wS6yNs4 4AH1MiVN02GroAcd0VhCfWkrYXg+e1lGUqUcVdt/6U/6m8sDSDXt+JTTjKKTx3YD3jZ6 Cmqw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1761283015; x=1761887815; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=1JwAfOvfHWiveHwG3B+VPqzu+ErYo2MF7kqHuEps3qs=; b=eot5XR57Hx0aoj4+SXrzZDum+ZoV7T4+6UISW+3Z40S5LSBxMie0EZnuaH8G7+ReHq uauOFfQtGOPSBi3QU2xQkv1BSXkiChu8laaaMbvI0lVa6Jfl+H0RxXWR4y0ds8FAuwIa Md269w0H9GEI6gZiUYhyGXYzjsD0o+2vhk+FlY2qVQ1jYwH2oBQb1sbsRfVABcuzJolK gNLVIqOoqzhb8UVnVHrEc8YQvHdtPasf8nyuwlhuCCuR+5rlEqgPK/869eUmySMEQdd0 r8y+Kghd0GhEatsbCmN9viLZOqMe0hKbbmIAa9zjZeGr4w2/Q+uUqDAJO/DvpGpH5XLO xYig== X-Forwarded-Encrypted: i=1; AJvYcCUE/Fu+0nNCW/Cgopr79MqucX7G8txOg2HICZKxh1M6OP52AUQu1Umx9wEuBNA0bxDfBfMcoDRuKOc7Yl4=@lists.ozlabs.org X-Gm-Message-State: AOJu0YyowqUEq2wrppXb8SZ+JItwcL5AbEk+P+FHhUARlX5AIFgA7Bjh IWcvKAzId1su5Dpw1n4dGDhnUyqHbgCeU1hDo0bPOYc3DAE4hJC4PvvqB8lgxeKhTVHZexwwVbW FryAgaw== X-Google-Smtp-Source: AGHT+IFxT94+rQGdQ1s8VDmzk/KuJ8lt96XLnKSdy7VNhFGvy0hrjNSBNuZhWw/inI+IBcmkkRvS1vw5Cn4= X-Received: from pjob8.prod.google.com ([2002:a17:90a:8c88:b0:33b:51fe:1a7a]) (user=kuniyu job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90a:d610:b0:33b:da53:d116 with SMTP id 98e67ed59e1d1-33fd660fe13mr1382618a91.26.1761283014686; Thu, 23 Oct 2025 22:16:54 -0700 (PDT) Date: Fri, 24 Oct 2025 05:16:42 +0000 In-Reply-To: <0bfa4895-727b-407b-90d2-7d54b9bd4910@intel.com> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list Mime-Version: 1.0 References: <0bfa4895-727b-407b-90d2-7d54b9bd4910@intel.com> X-Mailer: git-send-email 2.51.1.851.g4ebd6896fd-goog Message-ID: <20251024051653.66329-1-kuniyu@google.com> Subject: Re: [PATCH v1 2/2] epoll: Use __user_write_access_begin() and unsafe_put_user() in epoll_put_uevent(). From: Kuniyuki Iwashima To: dave.hansen@intel.com Cc: alex@ghiti.fr, aou@eecs.berkeley.edu, axboe@kernel.dk, bp@alien8.de, brauner@kernel.org, catalin.marinas@arm.com, christophe.leroy@csgroup.eu, dave.hansen@linux.intel.com, edumazet@google.com, hpa@zytor.com, kuni1840@gmail.com, kuniyu@google.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, maddy@linux.ibm.com, mingo@redhat.com, mpe@ellerman.id.au, npiggin@gmail.com, palmer@dabbelt.com, pjw@kernel.org, tglx@linutronix.de, torvalds@linux-foundation.org, will@kernel.org, x86@kernel.org Content-Type: text/plain; charset="UTF-8" From: Dave Hansen Date: Thu, 23 Oct 2025 12:40:59 -0700 > On 10/22/25 17:04, Kuniyuki Iwashima wrote: > > --- a/include/linux/eventpoll.h > > +++ b/include/linux/eventpoll.h > > @@ -82,11 +82,14 @@ static inline struct epoll_event __user * > > epoll_put_uevent(__poll_t revents, __u64 data, > > struct epoll_event __user *uevent) > > { > > - if (__put_user(revents, &uevent->events) || > > - __put_user(data, &uevent->data)) > > - return NULL; > > - > > - return uevent+1; > > + __user_write_access_begin(uevent, sizeof(*uevent)); > > + unsafe_put_user(revents, &uevent->events, efault); > > + unsafe_put_user(data, &uevent->data, efault); > > + user_access_end(); > > + return uevent + 1; > > +efault: > > + user_access_end(); > > + return NULL; > > } > > #endif > > This makes me nervous. The access_ok() check is quite a distance away. > I'd kinda want to see some performance numbers before doing this. Is > removing a single access_ok() even measurable? I noticed I made a typo in commit message, s/tcp_rr/udp_rr/. epoll_put_uevent() can be called multiple times in a single epoll_wait(), and we can see 1.7% more pps on UDP even when 1 thread has 1000 sockets only: server: $ udp_rr --nolog -6 -F 1000 -T 1 -l 3600 client: $ udp_rr --nolog -6 -F 1000 -T 256 -l 3600 -c -H $SERVER server: $ nstat > /dev/null; sleep 10; nstat | grep -i udp Without patch (2 stac/clac): Udp6InDatagrams 2205209 0.0 With patch (1 stac/clac): Udp6InDatagrams 2242602 0.0 >>> 2242602 / 2205209 * 100 101.6956669413194 I also took a microbenchmark with bpftrace and we can see more invocations of ep_try_send_events_ns() finish faster, and 4% more total calls: $ sudo bpftrace -e ' k:ep_try_send_events { @start[cpu] = nsecs; } kr:ep_try_send_events { if (@start[cpu]) { $delay = nsecs - @start[cpu]; delete(@start[cpu]); @ep_try_send_events_ns = hist($delay); } } END { clear(@start); }' -c 'sleep 10' Without patch: @ep_try_send_events_ns: [256, 512) 2483257 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| [512, 1K) 850735 |@@@@@@@@@@@@@@@@@ | [1K, 2K) 254027 |@@@@@ | [2K, 4K) 26646 | | [4K, 8K) 1358 | | [8K, 16K) 66 | | [16K, 32K) 3 | | With patch: @ep_try_send_events_ns: [256, 512) 2844733 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| [512, 1K) 733956 |@@@@@@@@@@@@@ | [1K, 2K) 166349 |@@@ | [2K, 4K) 13495 | | [4K, 8K) 526 | | [8K, 16K) 63 | | [16K, 32K) 5 | | >>> (2844733 + 733956 + 166349 + 13495 + 526 + 63 + 5) / \ ... (2483257 + 850735 + 254027 + 26646 + 1358 + 66 + 3) * 100 103.95551329999347 > > Also, even if we go do this, shouldn't __user_write_access_begin() be > called something more like unsafe_user_write_access_begin()? Sounds good.