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Wed, 28 Apr 2021 08:58:31 +0000 (GMT) Received: from b06wcsmtp001.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 258EAA4054; Wed, 28 Apr 2021 08:58:30 +0000 (GMT) Received: from oc7455500831.ibm.com (unknown [9.171.77.184]) by b06wcsmtp001.portsmouth.uk.ibm.com (Postfix) with ESMTP; Wed, 28 Apr 2021 08:58:30 +0000 (GMT) Subject: Re: sched: Move SCHED_DEBUG sysctl to debugfs From: Christian Borntraeger To: Peter Zijlstra Cc: bristot@redhat.com, bsegall@google.com, dietmar.eggemann@arm.com, greg@kroah.com, gregkh@linuxfoundation.org, joshdon@google.com, juri.lelli@redhat.com, linux-kernel@vger.kernel.org, linux@rasmusvillemoes.dk, mgorman@suse.de, mingo@kernel.org, rostedt@goodmis.org, valentin.schneider@arm.com, vincent.guittot@linaro.org, linux-s390@vger.kernel.org, kvm@vger.kernel.org References: <20210412102001.287610138@infradead.org> <20210427145925.5246-1-borntraeger@de.ibm.com> Message-ID: <49af1ed4-26b1-071e-365e-f701edb0eaa7@de.ibm.com> Date: Wed, 28 Apr 2021 10:58:24 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.9.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: E0iWOp_TzVfk6sbhp_bBZP4Aub___Fe8 X-Proofpoint-ORIG-GUID: pfPkjigkQPyf-rjhr3fzYERBEs9-9qHU X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.391,18.0.761 definitions=2021-04-28_03:2021-04-27,2021-04-28 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 adultscore=0 mlxscore=0 clxscore=1015 suspectscore=0 spamscore=0 impostorscore=0 phishscore=0 mlxlogscore=999 malwarescore=0 lowpriorityscore=0 priorityscore=1501 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2104060000 definitions=main-2104280057 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 28.04.21 10:54, Christian Borntraeger wrote: > > > On 28.04.21 10:46, Peter Zijlstra wrote: >> On Tue, Apr 27, 2021 at 04:59:25PM +0200, Christian Borntraeger wrote: >>> Peter, >>> >>> I just realized that we moved away sysctl tunabled to debugfs in next. >>> We have seen several cases where it was benefitial to set >>> sched_migration_cost_ns to a lower value. For example with KVM I can >>> easily get 50% more transactions with 50000 instead of 500000. >>> Until now it was possible to use tuned or /etc/sysctl.conf to set >>> these things permanently. >>> >>> Given that some people do not want to have debugfs mounted all the time >>> I would consider this a regression. The sysctl tunable was always >>> available. >>> >>> I am ok with the "informational" things being in debugfs, but not >>> the tunables. So how do we proceed here? >> >> It's all SCHED_DEBUG; IOW you're relying on DEBUG infrastructure for >> production performance, and that's your fail. > > No its not. sched_migration_cost_ns was NEVER protected by CONFIG_SCHED_DEBUG. > It was available on all kernels with CONFIG_SMP. Have to correct myself, it was SCHED_DEBUG in 3.0. > >> >> I very explicitly do not care to support people that poke random values >> into those 'tunables'. If people wants to do that, they get to keep any >> and all pieces. >> >> The right thing to do here is to analyze the situation and determine why >> migration_cost needs changing; is that an architectural thing, does s390 >> benefit from less sticky tasks due to its cache setup (the book caches >> could be absorbing some of the penalties here for example). Or is it >> something that's workload related, does KVM intrinsically not care about >> migrating so much, or is it something else. >> >> Basically, you get to figure out what the actual performance issue is, >> and then we can look at what to do about it so that everyone benefits, >> and not grow some random tweaks on the interweb that might or might not >> actually work for someone else. > > Yes, I agree. We have seen the effect of this value recently and we want > look into that. Still that does not change the fact that you are removing > an interface that was there for ages.