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Tue, 05 May 2020 17:30:37 +0000 Received: from d06av25.portsmouth.uk.ibm.com (d06av25.portsmouth.uk.ibm.com [9.149.105.61]) by b06avi18878370.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 045HUZou57868652 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 5 May 2020 17:30:35 GMT Received: from d06av25.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 6B4A311C058; Tue, 5 May 2020 17:30:35 +0000 (GMT) Received: from d06av25.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id BBBB711C04A; Tue, 5 May 2020 17:30:32 +0000 (GMT) Received: from oc7455500831.ibm.com (unknown [9.145.49.139]) by d06av25.portsmouth.uk.ibm.com (Postfix) with ESMTP; Tue, 5 May 2020 17:30:32 +0000 (GMT) Subject: Re: [PATCH v2 0/5] Statsfs: a new ram-based file sytem for Linux kernel statistics To: Paolo Bonzini , David Rientjes Cc: Jim Mattson , Emanuele Giuseppe Esposito , Jonathan Adams , kvm list , David Hildenbrand , Cornelia Huck , Vitaly Kuznetsov , Alexander Viro , Emanuele Giuseppe Esposito , LKML , linux-mips@vger.kernel.org, kvm-ppc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, Linux FS Devel , Stefan Raspl References: <20200504110344.17560-1-eesposit@redhat.com> <1d12f846-bf89-7b0a-5c71-e61d83b1a36f@redhat.com> <6cfdf81f-caef-2489-0906-25915d9d58ff@redhat.com> From: Christian Borntraeger Autocrypt: addr=borntraeger@de.ibm.com; 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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <6cfdf81f-caef-2489-0906-25915d9d58ff@redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Language: en-US Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.138,18.0.676 definitions=2020-05-05_09:2020-05-04,2020-05-05 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 bulkscore=0 spamscore=0 lowpriorityscore=0 clxscore=1011 mlxscore=0 mlxlogscore=999 impostorscore=0 adultscore=0 suspectscore=0 phishscore=0 malwarescore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2003020000 definitions=main-2005050131 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Adding Stefan Raspl, who has done a lot of kvm_stat work in the past. On 05.05.20 19:21, Paolo Bonzini wrote: > On 05/05/20 19:07, David Rientjes wrote: >>> I am totally in favor of having a binary format, but it should be >>> introduced as a separate series on top of this one---and preferably by >>> someone who has already put some thought into the problem (which >>> Emanuele and I have not, beyond ensuring that the statsfs concept and >>> API is flexible enough). >>> >> The concern is that once this series is merged then /sys/kernel/stats >> could be considered an ABI and there would be a reasonable expectation >> that it will remain stable, in so far as the stats that userspace is >> interested in are stable and not obsoleted. >> >> So is this a suggestion that the binary format becomes complementary to >> statsfs and provide a means for getting all stats from a single subsystem, >> or that this series gets converted to such a format before it is merged? > > The binary format should be complementary. The ASCII format should > indeed be considered stable even though individual statistics would come > and go. It may make sense to allow disabling ASCII files via mount > and/or Kconfig options; but either way, the binary format can and should > be added on top. > > I have not put any thought into what the binary format would look like > and what its features would be. For example these are but the first > questions that come to mind: > > * would it be possible to read/clear an arbitrary statistic with > pread/pwrite, or do you have to read all of them? > > * if userspace wants to read the schema just once and then read the > statistics many times, how is it informed of schema changes? > > * and of course the details of how the schema (names of stat and > subsources) is encoded and what details it should include about the > values (e.g. type or just signedness). > > Another possibility is to query stats via BPF. This could be a third > way to access the stats, or it could be alternative to a binary format. > > Paolo >