From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-179.mta1.migadu.com (out-179.mta1.migadu.com [95.215.58.179]) (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 3ED5126E6F6; Wed, 22 Oct 2025 16:00:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.179 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761148839; cv=none; b=jHnUD6qhjAACbCYsu/TdYq0F1Ka6dkhJGQEUmEeAjJC1nGZeNieC7MVqSO1g2Digs4p7b7K9n7uvXjMhPcxWBGHwsuqvCyEVq8b1L58oBipS4vzOYNZT59cvXXcTyAwi3DwxVrplIxNK5UAlqs2DHyDZ6XiFsJchz8ou94FTk4A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1761148839; c=relaxed/simple; bh=jbu7M00koy29d91adFhRysiT6pdG6JgiTtHNdmMfJek=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=a0gefolwTmYmQBa2C8IWalj0AY460QTEacf4pTkBly14ArS9v5iQIYnOV+wxZ40oPxqH3QTqWLgEFwy+lMQQXkkij5+hK5WvdlxwH00iRxDxtieq/rTs2eaO7e9IOvVSTXSwX7+6/pfJKHJhmDJOa8pgyzla2rP8Dti01HRI8JQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=qC7J67qH; arc=none smtp.client-ip=95.215.58.179 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="qC7J67qH" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1761148822; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Yp2cQvFiaIaAz+JhdGJjrvgCDsYO80NbgcjJ/2JWtSU=; b=qC7J67qHBbXFf8znPbB+TkYYl2uLEniHZ8byowwo+150h34q6YJrsdpYOPXzJH1SK1gl5f /uXLBaRH4Hh0gyxLtH1U2Qbw2ZD8PoTqVOSRQmyU54JlcB4aDjTZJ4ThdwKaNZKFs8tPqv 5O3cThVkFDvku8dHLxbEAWZu4x6tHfA= Date: Wed, 22 Oct 2025 23:59:32 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH bpf-next v3 2/2] bpf: Use per-cpu BPF callchain entry to save callchain To: Peter Zijlstra Cc: mingo@redhat.com, acme@kernel.org, namhyung@kernel.org, mark.rutland@arm.com, alexander.shishkin@linux.intel.com, jolsa@kernel.org, irogers@google.com, adrian.hunter@intel.com, kan.liang@linux.intel.com, song@kernel.org, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev, eddyz87@gmail.com, yonghong.song@linux.dev, john.fastabend@gmail.com, kpsingh@kernel.org, sdf@fomichev.me, haoluo@google.com, linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org References: <20251019170118.2955346-1-chen.dylane@linux.dev> <20251019170118.2955346-3-chen.dylane@linux.dev> <20251020110303.GS3419281@noisy.programming.kicks-ass.net> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Tao Chen In-Reply-To: <20251020110303.GS3419281@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 在 2025/10/20 19:03, Peter Zijlstra 写道: > On Mon, Oct 20, 2025 at 01:01:18AM +0800, Tao Chen wrote: >> As Alexei noted, get_perf_callchain() return values may be reused >> if a task is preempted after the BPF program enters migrate disable >> mode. Drawing on the per-cpu design of bpf_bprintf_buffers, >> per-cpu BPF callchain entry is used here. > > And now you can only unwind 3 tasks, and then start failing. This is > acceptable, why? Yes it is, if we use per-cpu-bpf-callchain-entry like bpf_bprintf_buffers, this is a proposal from Andrii and Alexei, In my understanding, is it a low-probability event to be preempted three times in a row in the same cpu? > >> - if (may_fault) >> - rcu_read_lock(); /* need RCU for perf's callchain below */ >> - > > I know you propose to remove this code; but how was that correct? The > perf callchain code hard relies on non-preemptible context, RCU does not > imply such a thing. > Alexei mentioned this rcu-lock issue before， It seems we need preemption protection. https://lore.kernel.org/bpf/CAADnVQ+s8B7-fvR1TNO-bniSyKv57cH_ihRszmZV7pQDyV=VDQ@mail.gmail.com >> if (trace_in) >> trace = trace_in; >> - else if (kernel && task) >> trace = get_callchain_entry_for_task(task, max_depth); >> - else >> - trace = get_perf_callchain(regs, NULL, kernel, user, max_depth, >> - crosstask, false); > > -- Best Regards Tao Chen