From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtpout.efficios.com (smtpout.efficios.com [167.114.26.122]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2CAC81C683; Mon, 9 Sep 2024 15:12:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=167.114.26.122 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725894725; cv=none; b=EOswOc9siYnk1tNLiCagJGH2HNtrJ6nVzVBeecZAycuPn2p7WgHql5eA1JAgOlzr6J/ZPWC/czLZXQ+8X7RgGoAAtSH6qy02ctfoxK0UjX1kK2Sx5/3py1rKngQ8HQtCYerEUpJ23LxF8J+Ad9leWeauEm+nno6qiTMFHPloTko= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725894725; c=relaxed/simple; bh=c7YvUH84AlumiDtIrio1hThdvteHMEAg8VTnwNP3vHc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Xuk+KOp8PeZYwDMxJiSodXy/EBZmP+jpCxwYEdQlalNj+G+X+lHRh75DwzPa9yWHo/IuNx8hu+4dPdNyeUZYHLU3h/vOxElHweNk4yrrXwvOZj+37IzO2JL2jd/U+hi3eEb3fuX7pw9B0R2/ivR1DxRcEOtSnCA4YWic4IGOB/g= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=efficios.com; spf=pass smtp.mailfrom=efficios.com; dkim=pass (2048-bit key) header.d=efficios.com header.i=@efficios.com header.b=Sc/IMzhX; arc=none smtp.client-ip=167.114.26.122 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=efficios.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=efficios.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=efficios.com header.i=@efficios.com header.b="Sc/IMzhX" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=efficios.com; s=smtpout1; t=1725894714; bh=c7YvUH84AlumiDtIrio1hThdvteHMEAg8VTnwNP3vHc=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Sc/IMzhXLu36YaYhkAE3zpwxZxsmgRydJSMBgXazLABBcFdivkUQcX/Nky1m9Okst 2cPsii6n9FN5ymmfrlOZ6FmTdZvBmD9+eReLdo396pjv8WMvPAmkStNH/pNWehd9Gl K7ibQ1yB2kUfmhK1E3d7uphByCYfVooQWr7qmIM9soWZnkOaE/oD6e3CErp6CZFjb2 xGffIACes2q1NoEm0Yn9iZCFh70omRIpJpWB1ShoZ4ToC0FkUPnlqRUjamtiqt24c+ aC7vWHqojzbsw5uudYx9+/iz/GuvSakBt5tRX5IvUm3fus3SUx38BGoyxuZMo6oQ6g TNRzodG4zDgeg== Received: from [172.16.0.134] (96-127-217-162.qc.cable.ebox.net [96.127.217.162]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4X2Vfx67y3z1KWR; Mon, 9 Sep 2024 11:11:53 -0400 (EDT) Message-ID: <1f442f99-92cd-41d6-8dd2-1f4780f2e556@efficios.com> Date: Mon, 9 Sep 2024 11:11:39 -0400 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 3/5] tracing/bpf-trace: Add support for faultable tracepoints To: Andrii Nakryiko Cc: Steven Rostedt , Masami Hiramatsu , linux-kernel@vger.kernel.org, Peter Zijlstra , Alexei Starovoitov , Yonghong Song , "Paul E . McKenney" , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Namhyung Kim , bpf@vger.kernel.org, Joel Fernandes , linux-trace-kernel@vger.kernel.org, Michael Jeanson References: <20240828144153.829582-1-mathieu.desnoyers@efficios.com> <20240828144153.829582-4-mathieu.desnoyers@efficios.com> Content-Language: en-US From: Mathieu Desnoyers In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 2024-09-04 21:21, Andrii Nakryiko wrote: > On Wed, Aug 28, 2024 at 7:42 AM Mathieu Desnoyers > wrote: >> >> In preparation for converting system call enter/exit instrumentation >> into faultable tracepoints, make sure that bpf can handle registering to >> such tracepoints by explicitly disabling preemption within the bpf >> tracepoint probes to respect the current expectations within bpf tracing >> code. >> >> This change does not yet allow bpf to take page faults per se within its >> probe, but allows its existing probes to connect to faultable >> tracepoints. >> >> Link: https://lore.kernel.org/lkml/20231002202531.3160-1-mathieu.desnoyers@efficios.com/ >> Co-developed-by: Michael Jeanson >> Signed-off-by: Mathieu Desnoyers >> Signed-off-by: Michael Jeanson >> Reviewed-by: Masami Hiramatsu (Google) >> Cc: Steven Rostedt >> Cc: Masami Hiramatsu >> Cc: Peter Zijlstra >> Cc: Alexei Starovoitov >> Cc: Yonghong Song >> Cc: Paul E. McKenney >> Cc: Ingo Molnar >> Cc: Arnaldo Carvalho de Melo >> Cc: Mark Rutland >> Cc: Alexander Shishkin >> Cc: Namhyung Kim >> Cc: bpf@vger.kernel.org >> Cc: Joel Fernandes >> --- >> Changes since v4: >> - Use DEFINE_INACTIVE_GUARD. >> - Add brackets to multiline 'if' statements. >> Changes since v5: >> - Rebased on v6.11-rc5. >> - Pass the TRACEPOINT_MAY_FAULT flag directly to tracepoint_probe_register_prio_flags. >> --- >> include/trace/bpf_probe.h | 21 ++++++++++++++++----- >> kernel/trace/bpf_trace.c | 2 +- >> 2 files changed, 17 insertions(+), 6 deletions(-) >> >> diff --git a/include/trace/bpf_probe.h b/include/trace/bpf_probe.h >> index a2ea11cc912e..cc96dd1e7c3d 100644 >> --- a/include/trace/bpf_probe.h >> +++ b/include/trace/bpf_probe.h >> @@ -42,16 +42,27 @@ >> /* tracepoints with more than 12 arguments will hit build error */ >> #define CAST_TO_U64(...) CONCATENATE(__CAST, COUNT_ARGS(__VA_ARGS__))(__VA_ARGS__) >> >> -#define __BPF_DECLARE_TRACE(call, proto, args) \ >> +#define __BPF_DECLARE_TRACE(call, proto, args, tp_flags) \ >> static notrace void \ >> __bpf_trace_##call(void *__data, proto) \ >> { \ >> - CONCATENATE(bpf_trace_run, COUNT_ARGS(args))(__data, CAST_TO_U64(args)); \ >> + DEFINE_INACTIVE_GUARD(preempt_notrace, bpf_trace_guard); \ >> + \ >> + if ((tp_flags) & TRACEPOINT_MAY_FAULT) { \ >> + might_fault(); \ >> + activate_guard(preempt_notrace, bpf_trace_guard)(); \ >> + } \ >> + \ >> + CONCATENATE(bpf_trace_run, COUNT_ARGS(args))(__data, CAST_TO_U64(args)); \ >> } >> >> #undef DECLARE_EVENT_CLASS >> #define DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print) \ >> - __BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args)) >> + __BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args), 0) >> + >> +#undef DECLARE_EVENT_CLASS_MAY_FAULT >> +#define DECLARE_EVENT_CLASS_MAY_FAULT(call, proto, args, tstruct, assign, print) \ >> + __BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args), TRACEPOINT_MAY_FAULT) >> >> /* >> * This part is compiled out, it is only here as a build time check >> @@ -105,13 +116,13 @@ static inline void bpf_test_buffer_##call(void) \ >> >> #undef DECLARE_TRACE >> #define DECLARE_TRACE(call, proto, args) \ >> - __BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args)) \ >> + __BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args), 0) \ >> __DEFINE_EVENT(call, call, PARAMS(proto), PARAMS(args), 0) >> >> #undef DECLARE_TRACE_WRITABLE >> #define DECLARE_TRACE_WRITABLE(call, proto, args, size) \ >> __CHECK_WRITABLE_BUF_SIZE(call, PARAMS(proto), PARAMS(args), size) \ >> - __BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args)) \ >> + __BPF_DECLARE_TRACE(call, PARAMS(proto), PARAMS(args), 0) \ >> __DEFINE_EVENT(call, call, PARAMS(proto), PARAMS(args), size) >> >> #include TRACE_INCLUDE(TRACE_INCLUDE_FILE) >> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c >> index c77eb80cbd7f..ed07283d505b 100644 >> --- a/kernel/trace/bpf_trace.c >> +++ b/kernel/trace/bpf_trace.c >> @@ -2473,7 +2473,7 @@ int bpf_probe_register(struct bpf_raw_event_map *btp, struct bpf_raw_tp_link *li >> >> return tracepoint_probe_register_prio_flags(tp, (void *)btp->bpf_func, >> link, TRACEPOINT_DEFAULT_PRIO, >> - TRACEPOINT_MAY_EXIST); >> + TRACEPOINT_MAY_EXIST | (tp->flags & TRACEPOINT_MAY_FAULT)); >> } >> >> int bpf_probe_unregister(struct bpf_raw_event_map *btp, struct bpf_raw_tp_link *link) >> -- >> 2.39.2 >> >> > > I wonder if it would be better to just do this, instead of that > preempt guard. I think we don't strictly need preemption to be > disabled, we just need to stay on the same CPU, just like we do that > for many other program types. I'm worried about introducing any kind of subtle synchronization change in this series, and moving from preempt-off to migrate-disable definitely falls under that umbrella. I would recommend auditing all uses of this_cpu_*() APIs to make sure accesses to per-cpu data structures are using atomics and not just using operations that expect use of preempt-off to prevent concurrent threads from updating to the per-cpu data concurrently. So what you are suggesting may be a good idea, but I prefer to leave this kind of change to a separate bpf-specific series, and I would leave this work to someone who knows more about ebpf than me. Thanks, Mathieu > > We'll need some more BPF-specific plumbing to fully support faultable > (sleepable) tracepoints, but this should unblock your work, unless I'm > missing something. And we can take it from there, once your patches > land, to take advantage of faultable tracepoints in the BPF ecosystem. > > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c > index b69a39316c0c..415639b7c7a4 100644 > --- a/kernel/trace/bpf_trace.c > +++ b/kernel/trace/bpf_trace.c > @@ -2302,7 +2302,8 @@ void __bpf_trace_run(struct bpf_raw_tp_link > *link, u64 *args) > struct bpf_run_ctx *old_run_ctx; > struct bpf_trace_run_ctx run_ctx; > > - cant_sleep(); > + migrate_disable(); > + > if (unlikely(this_cpu_inc_return(*(prog->active)) != 1)) { > bpf_prog_inc_misses_counter(prog); > goto out; > @@ -2318,6 +2319,8 @@ void __bpf_trace_run(struct bpf_raw_tp_link > *link, u64 *args) > bpf_reset_run_ctx(old_run_ctx); > out: > this_cpu_dec(*(prog->active)); > + > + migrate_enable(); > } -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com