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 073918C06; Fri, 4 Oct 2024 01:35:19 +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=1728005720; cv=none; b=AZ0Oy6lo9FY0SwZl1MElIwLp6ig+tULvJlzaYJKkoIseW2DsmUHokv5IZe4dljemwOGvO/W1Yrkpuc5iMFgvNUGdhmHBmz8IIwt3j6vLgJYrOhuZbPJ0lhneKZtW3hUXj71AOZMFJ5kYQJ5tTCX9Z4Pjx3t3vOrTHR5sIryHY0Y= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728005720; c=relaxed/simple; bh=kFIgYd1GyoFetX5O9S6+TQCvwyv29zV3/01SyhjdM48=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=c5UFu2Bc3eAg5Worhss9fwd5j6PzzdzyzxqHR8VNJ9sNAU7CmlQL/S2XMGVWYhT3IDSz/d1Uv5uw4MXdlTPTd2rFakmEuT4NhoFgnEaToOAChCZM4wNq+ipkBywDAWyRx5cJ19XBQS9pbqQCUkAKXedx7JLVL3XPeyx5k8klnBE= 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=eeCr3bhs; 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="eeCr3bhs" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=efficios.com; s=smtpout1; t=1728005718; bh=kFIgYd1GyoFetX5O9S6+TQCvwyv29zV3/01SyhjdM48=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=eeCr3bhsVEgaQxQU4Q/ZasrFWXoCTlqtwLQDd6Cpma6aE+Y86bQtR8C+Z3NJVb3Bc yus2m6gyQsjXOgASgtK3LwdPrkE6LmNrAirKEFxX6LO7SoS0AlMfCba/IhnIQKQwWT TWVq0ttmUKjGIOQMlSMHgzxVHuBhi+VF864vtU0qRnhikSVTbnW7WKqkvoZyUwFNFx baFDHDU0zPqpBB4IJeQmufWq4KcjcDPxUN9+ypCzD43rc+ypBYAuxkYU2Cn6pPT8W+ IngmVuoEEz27Yb055/850UDkGMyKogUdfmUv48Y7Lt5NnJK7x/AmG3b8EvjPV0eRhA q8aMMxvO0sOKA== Received: from [IPV6:2606:6d00:100:4000:cacb:9855:de1f:ded2] (unknown [IPv6:2606:6d00:100:4000:cacb:9855:de1f:ded2]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4XKWM960NSzC73; Thu, 3 Oct 2024 21:35:17 -0400 (EDT) Message-ID: <90ca2fee-cdfb-4d48-ab9e-57d8d2b8b8d8@efficios.com> Date: Thu, 3 Oct 2024 21:33:16 -0400 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v1 2/8] tracing/ftrace: guard syscall probe with preempt_notrace To: Steven Rostedt Cc: 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 , Andrii Nakryiko , bpf@vger.kernel.org, Joel Fernandes , linux-trace-kernel@vger.kernel.org, Michael Jeanson References: <20241003151638.1608537-1-mathieu.desnoyers@efficios.com> <20241003151638.1608537-3-mathieu.desnoyers@efficios.com> <20241003182304.2b04b74a@gandalf.local.home> <6dc21f67-52e1-4ed5-af7f-f047c3c22c11@efficios.com> <20241003210403.71d4aa67@gandalf.local.home> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: <20241003210403.71d4aa67@gandalf.local.home> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2024-10-04 03:04, Steven Rostedt wrote: > On Thu, 3 Oct 2024 20:26:29 -0400 > Mathieu Desnoyers wrote: > > >> static void ftrace_syscall_enter(void *data, struct pt_regs *regs, long id) >> { >> struct trace_array *tr = data; >> struct trace_event_file *trace_file; >> struct syscall_trace_enter *entry; >> struct syscall_metadata *sys_data; >> struct trace_event_buffer fbuffer; >> unsigned long args[6]; >> int syscall_nr; >> int size; >> >> syscall_nr = trace_get_syscall_nr(current, regs); >> if (syscall_nr < 0 || syscall_nr >= NR_syscalls) >> return; >> >> /* Here we're inside tp handler's rcu_read_lock_sched (__DO_TRACE) */ >> trace_file = rcu_dereference_sched(tr->enter_syscall_files[syscall_nr]); >> >> ^^^^ this function explicitly states that preempt needs to be disabled by >> tracepoints. > > Ah, I should have known it was the syscall portion. I don't care for this > hidden dependency. I rather add a preempt disable here and not expect it to > be disabled when called. Which is exactly what this patch is doing. > >> >> if (!trace_file) >> return; >> >> if (trace_trigger_soft_disabled(trace_file)) >> return; >> >> sys_data = syscall_nr_to_meta(syscall_nr); >> if (!sys_data) >> return; >> >> size = sizeof(*entry) + sizeof(unsigned long) * sys_data->nb_args; >> >> entry = trace_event_buffer_reserve(&fbuffer, trace_file, size); >> ^^^^ it reserves space in the ring buffer without disabling preemption explicitly. >> >> And also: >> >> void *trace_event_buffer_reserve(struct trace_event_buffer *fbuffer, >> struct trace_event_file *trace_file, >> unsigned long len) >> { >> struct trace_event_call *event_call = trace_file->event_call; >> >> if ((trace_file->flags & EVENT_FILE_FL_PID_FILTER) && >> trace_event_ignore_this_pid(trace_file)) >> return NULL; >> >> /* >> * If CONFIG_PREEMPTION is enabled, then the tracepoint itself disables >> * preemption (adding one to the preempt_count). Since we are >> * interested in the preempt_count at the time the tracepoint was >> * hit, we need to subtract one to offset the increment. >> */ >> ^^^ This function also explicitly expects preemption to be disabled. >> >> So I rest my case. The change I'm introducing for tracepoints >> don't make any assumptions about whether or not each tracer require >> preempt off or not: it keeps the behavior the _same_ as it was before. >> >> Then it's up to each tracer's developer to change the behavior of their >> own callbacks as they see fit. But I'm not introducing regressions in >> tracers with the "big switch" change of making syscall tracepoints >> faultable. This will belong to changes that are specific to each tracer. > > > I rather remove these dependencies at the source. So, IMHO, these places > should be "fixed" first. > > At least for the ftrace users. But I think the same can be done for the > other users as well. BPF already stated it just needs "migrate_disable()". > Let's see what perf has. > > We can then audit all the tracepoint users to make sure they do not need > preemption disabled. Why does it need to be a broad refactoring of the entire world ? What is wrong with the simple approach of introducing this tracepoint faultable syscall support as a no-op from the tracer's perspective ? Then we can build on top and figure out if we want to relax things on a tracer-per-tracer basis. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com