From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 4BE2A35A922; Tue, 6 Jan 2026 18:54:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767725700; cv=none; b=oExTBsxQxJDrYXqscEKj644nzrqGjtv4tYLWCP6gXr/8a/r0S0lIJvUKozTgnjMZCV2EQNNHtfW2D4EN/BZVOPyQscZJm2YZFvIDJGr8FHCOJjNjZIhmlvCfGM9cDX8pIDmYShUDVdAguR60aHCggBzGdEeJlrCDveWtLLPuO4o= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767725700; c=relaxed/simple; bh=OZ0lYvE27BoXnsUrCJis4sLZ9xgMFKfugHtLgCQtoPw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=HsRB7tPV64xQLBAknizBnxUe/8yEsQb6zA49FT8WGLFeNlOvHlJmwJ5y1SMU7Y5oOoNzeGJvwdyIs5lgUYWdgeqlrinkjrEBfe5P43hw5fxSt1SJiHNhdu1Oc5pAiUcB95GQ0zpL+g27ixl12AjaSHIZV6/epmxYmMKVs3thhZg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=G72D2UhT; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="G72D2UhT" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 480AFC116C6; Tue, 6 Jan 2026 18:54:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1767725698; bh=OZ0lYvE27BoXnsUrCJis4sLZ9xgMFKfugHtLgCQtoPw=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=G72D2UhTUxCMo8vfXKw5ttRU7tJkM8/o2pUVVSQ7XcUnfSmBjWCVSDGepNw9T6qvc cjbAMxSaHVkx5WHx321NiYIkVb18oZTU/DD5ExAnqNVO2hD/OYuJCw71e9KEo2vddE AkzDzNc8BQByj42ua4vomS1GYcaRYE3A4neOh0DyVJbH8ycYZ80TkH5mtoEj5ldwj5 aOg6uFnJlKZlio+b5AcFSPaSRCSgq+9M8qpZ+uyCrF8rHgZm++Gj7EAYfuOwXFn7F9 EU0ctFByAQoLKmMuwRDZbQVAeWBHIJrEpAkyF7KhjZRcUqLTzrZFgmdAbh0FeT9D9O PI/X6mFS40uHw== Message-ID: <175da76e-3f04-460b-8629-05062edd2d62@kernel.org> Date: Tue, 6 Jan 2026 19:54:54 +0100 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [v2 PATCH 1/1] fs/proc: Expose mm_cpumask in /proc/[pid]/status To: Aaron Tomlin Cc: oleg@redhat.com, akpm@linux-foundation.org, gregkh@linuxfoundation.org, brauner@kernel.org, mingo@kernel.org, sean@ashe.io, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org References: <20251226211407.2252573-1-atomlin@atomlin.com> <20251226211407.2252573-2-atomlin@atomlin.com> From: "David Hildenbrand (Red Hat)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 1/1/26 02:19, Aaron Tomlin wrote: > On Tue, Dec 30, 2025 at 10:16:30PM +0100, David Hildenbrand (Red Hat) wrote: >> Just a note: I have the faint recollection that there are some arch-specific >> oddities around mm_cpumask(). >> >> In particular, that some architectures never clear CPUs from the mask, while >> others (e.g., x86) clear them one the TLB for them is clean. >> >> I'd assume that all architectures at least set the CPUs once they ever ran >> an MM. But are we sure about that? >> >> $ git grep mm_cpumask | grep m68k >> >> gives me no results and I don't see common code to ever set a cpu in >> the mm_cpumask. >> >> -- >> Cheers >> > Hi David, > > You are correct; mm_cpumask semantics vary across architectures (e.g., arc) > and are even unused on some (e.g., m68k). > > Rather than attempting to standardise this across all architectures, I > propose we restrict this information to those that follow the "Lazy" TLB > model-specifically x86. In this model, the mask represents CPUs that might > hold stale TLB entries for a given MM and thus require IPI-based TLB > shootdowns to maintain coherency. Since this is the primary context where > mm_cpumask provides actionable debug data for performance bottlenecks, > showing it only for x86 (where it is reliably maintained) seems the most > pragmatic path. Yes, starting with a very restrictive set, and carefully documenting it sounds good to me. One question is what would happen if these semantics one day change on x86. I guess best we can do is to ... document it very carefully. > > I can document this arch-specific limitation in > Documentation/filesystems/proc.rst and wrapped the implementation in > CONFIG_X86 to avoid exposing "Best Effort" or zeroed-out data on > architectures where the mask is not meaningful. > > Please let me know your thoughts. Something along these lines. Maybe we want an CONFIG_ARCH_* define to unlock this from arch code. -- Cheers David