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 69CC8198A24; Thu, 10 Oct 2024 16:46:05 +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=1728578765; cv=none; b=DCMTkt/C2+8nbNEs3e6lM4nYBwee4HZrg78QMXhJrQDQrUbgdFtI+QHr00NivUKdFS9ZSEnFip8szwG3VjrKo9qwL4URCFaapwugseFqQqgYtsvinDm3l+B/EVHIEo8ipV4qoiKP0SeDNlhSnoeekI79a3RnGjh3K6w1wj4ctas= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1728578765; c=relaxed/simple; bh=08oJ1Ty/lrHxJqMnlNGf1Ch3DicmHiyZz4nCpw21uTY=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=PBIor1wkxdKFAH3I1qRrh+siVg2iwGUUowS+oxncSFB0AcESswULCjWEbiu5zbJoNAZmslgtCKgEIJ158Kx4NmGFCKZQAHxHNxaL/UdcyeE10ygvY8j9+sAJluDTvvCz+La3qujHKPu6Yl7NiMgld8xCTbcX+ZkOgnHyj9D7DMQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=u6Ix0bf3; 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="u6Ix0bf3" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2C578C4CEC5; Thu, 10 Oct 2024 16:46:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1728578765; bh=08oJ1Ty/lrHxJqMnlNGf1Ch3DicmHiyZz4nCpw21uTY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=u6Ix0bf3cc4FRzmp91azgETmOFuE1wfVYXinRRyp/GD+FEuglq4TSdz9649GSdcug 0SYmCsewJ/UiaKzoy9ydisL4moKN3CEBN2+blXvMpwfEJkE+N9G+GGLGgDu2kAUQQ7 GbIEOhxWfoQmkdZaloxwfgYsdCdfcdKI3l9c8rlRClxSNqJxVPvZJ3m9Ld5lmnQKFl 4eguRKzkJpu5ZhMzDRvgm/SoBWRz33hySRLia5MnJpQ5q62NT0FVOP0DGZbXm7QO1y Hlf3uE9SKbY6T/2PeGYHYsWHacXMGWVSUVEuKEF7Tbdb6IMv9M7epOv6zWu8rOn+om Nmzi5alsOz6RQ== Date: Thu, 10 Oct 2024 09:46:02 -0700 From: Namhyung Kim To: Vlastimil Babka Cc: Roman Gushchin , Song Liu , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Martin KaFai Lau , Eduard Zingerman , Yonghong Song , John Fastabend , KP Singh , Stanislav Fomichev , Hao Luo , Jiri Olsa , LKML , bpf@vger.kernel.org, Andrew Morton , Christoph Lameter , Pekka Enberg , David Rientjes , Joonsoo Kim , Hyeonggon Yoo <42.hyeyoo@gmail.com>, linux-mm@kvack.org, Arnaldo Carvalho de Melo , Kees Cook Subject: Re: [PATCH v4 bpf-next 2/3] mm/bpf: Add bpf_get_kmem_cache() kfunc Message-ID: References: <20241002180956.1781008-1-namhyung@kernel.org> <20241002180956.1781008-3-namhyung@kernel.org> <37ca3072-4a0b-470f-b5b2-9828a2b708e5@suse.cz> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Wed, Oct 09, 2024 at 12:17:12AM -0700, Namhyung Kim wrote: > On Mon, Oct 07, 2024 at 02:57:08PM +0200, Vlastimil Babka wrote: > > On 10/4/24 11:25 PM, Roman Gushchin wrote: > > > On Fri, Oct 04, 2024 at 01:10:58PM -0700, Song Liu wrote: > > >> On Wed, Oct 2, 2024 at 11:10 AM Namhyung Kim wrote: > > >>> > > >>> The bpf_get_kmem_cache() is to get a slab cache information from a > > >>> virtual address like virt_to_cache(). If the address is a pointer > > >>> to a slab object, it'd return a valid kmem_cache pointer, otherwise > > >>> NULL is returned. > > >>> > > >>> It doesn't grab a reference count of the kmem_cache so the caller is > > >>> responsible to manage the access. The intended use case for now is to > > >>> symbolize locks in slab objects from the lock contention tracepoints. > > >>> > > >>> Suggested-by: Vlastimil Babka > > >>> Acked-by: Roman Gushchin (mm/*) > > >>> Acked-by: Vlastimil Babka #mm/slab > > >>> Signed-off-by: Namhyung Kim > > > > > > So IIRC from our discussions with Namhyung and Arnaldo at LSF/MM I > > thought the perf use case was: > > > > - at the beginning it iterates the kmem caches and stores anything of > > possible interest in bpf maps or somewhere - hence we have the iterator > > - during profiling, from object it gets to a cache, but doesn't need to > > access the cache - just store the kmem_cache address in the perf record > > - after profiling itself, use the information in the maps from the first > > step together with cache pointers from the second step to calculate > > whatever is necessary > > Correct. > > > > > So at no point it should be necessary to take refcount to a kmem_cache? > > > > But maybe "bpf_get_kmem_cache()" is implemented here as too generic > > given the above use case and it should be implemented in a way that the > > pointer it returns cannot be used to access anything (which could be > > unsafe), but only as a bpf map key - so it should return e.g. an > > unsigned long instead? > > Yep, this should work for my use case. Maybe we don't need the > iterator when bpf_get_kmem_cache() kfunc returns the valid pointer as > we can get the necessary info at the moment. But I think it'd be less > efficient as more work need to be done at the event (lock contention). > It'd better setting up necessary info in a map before monitoring (using > the iterator), and just looking up the map with the kfunc while > monitoring the lock contention. Maybe it's still better to return a non-refcounted pointer for future use. I'll leave it for v5. Thanks, Namhyung