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X-MS-Exchange-CrossTenant-Id: 4e2c6054-71cb-48f1-bd6c-3a9705aca71b X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: GqFIoFQeiFh0QTuZORgfZYtVjt2y3m9Dsfg47fr3qivQBfhm3iX5cuDt8VXnzouJpuKd1NqvEWLwRtyfWPYM+A== X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM3PR10MB7972 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1099,Hydra:6.0.736,FMLib:17.12.80.40 definitions=2025-05-01_01,2025-04-24_02,2025-02-21_01 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 mlxlogscore=999 malwarescore=0 suspectscore=0 adultscore=0 phishscore=0 mlxscore=0 bulkscore=0 spamscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2504070000 definitions=main-2505010020 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwNTAxMDAyMSBTYWx0ZWRfXyTWyddRdWHKl QzYXHLR7HWKfn78YOQ6d3mL2Ogb320HvzQecPUvsGFNyVRYAeZ81kIeoA4WIxEbvkSZfatk8CbJ k5RL1QtzJqfC6f4fi+XJ7E/hh6PVzEZVC8/RlUnz2GqH64Db1YgjMWdhXmE9ymsD91QxhGH9BgV ON+LvoEJ3VRjrMKFqRqEw+2RUGbdAK25/wcinr7w03zqg/Kl+elFvwllDjvge7H2H0po2epA001 zdCU41iNabin6NXLFPF122YvCxQ5rWZdhMWvbKgimInuThk5DTvkYicYuE6zxrnAdMYpPPpRfvm nChVJ8Sdc0MQjGvSOCtSsLFRP66cZ8AH/goY3U1ovH+dLaBlcZaZUD3iTb0LMJ/YqlwA8ZcZqRg 2+/43NhCQWlgKEvA9g7zmD6zBX7z2mV0nzzWPiSVe7P58AanE28u1WoqNAJt8GJ+W9TWhzKP X-Proofpoint-GUID: q5Ml9isTA1XfHaAWGr6B0IvNX7qkMzG1 X-Authority-Analysis: v=2.4 cv=A5VsP7WG c=1 sm=1 tr=0 ts=6812e082 cx=c_pps a=XiAAW1AwiKB2Y8Wsi+sD2Q==:117 a=XiAAW1AwiKB2Y8Wsi+sD2Q==:17 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=wKuvFiaSGQ0qltdbU6+NXLB8nM8=:19 a=Ol13hO9ccFRV9qXi2t6ftBPywas=:19 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=dt9VzEwgFbYA:10 a=GoEa3M9JfhUA:10 a=XXPTK-dxorRYMydK5xEA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-ORIG-GUID: q5Ml9isTA1XfHaAWGr6B0IvNX7qkMzG1 Hi Prateek, On 4/30/25 04:29, K Prateek Nayak wrote: > Hello Libo, > > On 4/30/2025 4:11 PM, Libo Chen wrote: >> >> >> On 4/30/25 02:13, K Prateek Nayak wrote: >>> (+ more scheduler folks) >>> >>> tl;dr >>> >>> JB has a workload that hates aggressive migration on the 2nd Generation >>> EPYC platform that has a small LLC domain (4C/8T) and very noticeable >>> C2C latency. >>> >>> Based on JB's observation so far, reverting commit 16b0a7a1a0af >>> ("sched/fair: Ensure tasks spreading in LLC during LB") and commit >>> c5b0a7eefc70 ("sched/fair: Remove sysctl_sched_migration_cost >>> condition") helps the workload. Both those commits allow aggressive >>> migrations for work conservation except it also increased cache >>> misses which slows the workload quite a bit. >>> >>> "relax_domain_level" helps but cannot be set at runtime and I couldn't >>> think of any stable / debug interfaces that JB hasn't tried out >>> already that can help this workload. >>> >>> There is a patch towards the end to set "relax_domain_level" at >>> runtime but given cpusets got away with this when transitioning to >>> cgroup-v2, I don't know what the sentiments are around its usage. >>> Any input / feedback is greatly appreciated. >>> >> >> >> Hi Prateek, >> >> Oh no, not "relax_domain_level" again, this can lead to load imbalance >> in variety of ways. We were so glad this one went away with cgroupv2, > > I agree it is not pretty. JB also tried strategic pinning and they > did report that things are better overall but unfortunately, it is > very hard to deploy across multiple architectures and would also > require some redesign + testing from their application side. > I was more of stressing broadly how bad setting "relax_domain_level" could go wrong if an user doesn't know this essentially disables newidle balancing at higher levels, so the ability to balance loads across CCXes or NUMA nodes will be a lot weaker. A subset of CCXes may consistently get much more loads due to a whole bunch of reasons. Sometimes this is hard to spot in testing, but does show up in real-world scenarios, esp. when users have other weird hacks. >> it tends to be abused by users as an "easy" fix for some urgent perf >> issues instead of addressing their root causes. > > Was there ever a report of similar issue where migrations for right > reasons has led to performance degradation as a result of platform > architecture? I doubt there is a straightforward way to solve this > using the current interfaces - at least I haven't found one yet. > It wasn't due to platform architecture for us but more of "exotic" NUMA topology (like a cubic, a node is one hop away from 3 neighbors, two hops away from other 4) in combination with certain userlevel settings that cause more wakeups in a subset of domains. If relax_domain_level is left untouched, then you get no load imbalance but perf is bad. But once you set relax_domain_level to restrict newidle balancing to lower domain levels, you actually see better performance numbers in testing even though CPU loads are not well-balanced. Until one day, you find out the imbalance is so bad that it slows down everything. Luckily it wasn't too hard to fix from the application side. I get it may not be easy to fix from their application side in this case and but I still think this is too hackery, one may end up regretting. I certainly want to hear what others think about relax_domain_level! > Perhaps cache-aware scheduling is the way forward to solve these > set of issues as Peter highlighted. > Hope so! We will start test that series and provide feedback Thanks, Libo