Messages in this thread | | | Date | Tue, 20 Jun 2023 16:05:45 +0530 | Subject | Re: [tip: sched/core] sched: Fix performance regression introduced by mm_cid | From | Swapnil Sapkal <> |
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Hello Peter,
On 6/20/2023 2:41 PM, Peter Zijlstra wrote: > On Tue, Jun 20, 2023 at 01:44:32PM +0530, Swapnil Sapkal wrote: >> Hello Mathieu, >> >> On 4/22/2023 1:13 PM, tip-bot2 for Mathieu Desnoyers wrote: >>> The following commit has been merged into the sched/core branch of tip: >>> >>> Commit-ID: 223baf9d17f25e2608dbdff7232c095c1e612268 >>> Gitweb: https://git.kernel.org/tip/223baf9d17f25e2608dbdff7232c095c1e612268 >>> Author: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> >>> AuthorDate: Thu, 20 Apr 2023 10:55:48 -04:00 >>> Committer: Peter Zijlstra <peterz@infradead.org> >>> CommitterDate: Fri, 21 Apr 2023 13:24:20 +02:00 >>> >>> sched: Fix performance regression introduced by mm_cid >>> >>> Introduce per-mm/cpu current concurrency id (mm_cid) to fix a PostgreSQL >>> sysbench regression reported by Aaron Lu. >>> >>> Keep track of the currently allocated mm_cid for each mm/cpu rather than >>> freeing them immediately on context switch. This eliminates most atomic >>> operations when context switching back and forth between threads >>> belonging to different memory spaces in multi-threaded scenarios (many >>> processes, each with many threads). The per-mm/per-cpu mm_cid values are >>> serialized by their respective runqueue locks. >>> >>> Thread migration is handled by introducing invocation to >>> sched_mm_cid_migrate_to() (with destination runqueue lock held) in >>> activate_task() for migrating tasks. If the destination cpu's mm_cid is >>> unset, and if the source runqueue is not actively using its mm_cid, then >>> the source cpu's mm_cid is moved to the destination cpu on migration. >>> >>> Introduce a task-work executed periodically, similarly to NUMA work, >>> which delays reclaim of cid values when they are unused for a period of >>> time. >>> >>> Keep track of the allocation time for each per-cpu cid, and let the task >>> work clear them when they are observed to be older than >>> SCHED_MM_CID_PERIOD_NS and unused. This task work also clears all >>> mm_cids which are greater or equal to the Hamming weight of the mm >>> cidmask to keep concurrency ids compact. >>> >>> Because we want to ensure the mm_cid converges towards the smaller >>> values as migrations happen, the prior optimization that was done when >>> context switching between threads belonging to the same mm is removed, >>> because it could delay the lazy release of the destination runqueue >>> mm_cid after it has been replaced by a migration. Removing this prior >>> optimization is not an issue performance-wise because the introduced >>> per-mm/per-cpu mm_cid tracking also covers this more specific case. >>> >>> Fixes: af7f588d8f73 ("sched: Introduce per-memory-map concurrency ID") >>> Reported-by: Aaron Lu <aaron.lu@intel.com> >>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> >>> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> >>> Tested-by: Aaron Lu <aaron.lu@intel.com> >>> Link: https://lore.kernel.org/lkml/20230327080502.GA570847@ziqianlu-desk2/ >> >> I run standard benchmarks as a part of kernel performance regression >> testing. When I run these benchmarks against v6.3.0 to v6.4-rc1, >> I have seen performance regression in hackbench running with threads. When I did >> git bisect it pointed to this commit and reverting this commit helps regains >> the performance. This regression is not seen with hackbench processes. > > Well, *this* commit was supposed to help fix the horrible contention on > cid_lock that was introduced with af7f588d8f73.
I went back and tested the commit that introduced mm_cid and I found that the original implementation actually helped hackbench. Following are numbers from 2 Socket Zen3 Server (2 X 64C/128T):
Test: base (v6.2-rc1) base + orig_mm_cid 1-groups: 4.29 (0.00 pct) 4.32 (-0.69 pct) 2-groups: 4.96 (0.00 pct) 4.94 (0.40 pct) 4-groups: 5.21 (0.00 pct) 4.10 (21.30 pct) 8-groups: 5.44 (0.00 pct) 4.50 (17.27 pct) 16-groups: 7.09 (0.00 pct) 5.28 (25.52 pct)
I see following IBS traces in this case:
Base:
6.69% sched-messaging [kernel.vmlinux] [k] copy_user_generic_string 5.38% sched-messaging [kernel.vmlinux] [k] native_queued_spin_lock_slowpath 3.73% swapper [kernel.vmlinux] [k] __switch_to_asm 3.23% sched-messaging [kernel.vmlinux] [k] __calc_delta 2.93% sched-messaging [kernel.vmlinux] [k] try_to_wake_up 2.63% sched-messaging [kernel.vmlinux] [k] dequeue_task_fair 2.56% sched-messaging [kernel.vmlinux] [k] osq_lock
Base + orig_mm_cid:
13.70% sched-messaging [kernel.vmlinux] [k] native_queued_spin_lock_slowpath 11.87% swapper [kernel.vmlinux] [k] native_queued_spin_lock_slowpath 8.99% sched-messaging [kernel.vmlinux] [k] copy_user_generic_string 6.08% sched-messaging [kernel.vmlinux] [k] osq_lock 4.79% sched-messaging [kernel.vmlinux] [k] apparmor_file_permission 3.71% sched-messaging [kernel.vmlinux] [k] mutex_spin_on_owner 3.66% sched-messaging [kernel.vmlinux] [k] ktime_get_coarse_real_ts64 3.11% sched-messaging [kernel.vmlinux] [k] _copy_from_iter
> >> Following are the results from 1 Socket 4th generation EPYC >> Processor(1 X 96C/192T) configured in NPS1 mode. This regression >> becomes more severe as the number of core count increases. >> >> The numbers on a 1 Socket Bergamo (1 X 128 cores/256 threads) is significantly worse. >> >> Threads: >> >> Test: With-mmcid-patch Without-mmcid-patch >> 1-groups: 5.23 (0.00 pct) 4.61 (+11.85 pct) >> 2-groups: 4.99 (0.00 pct) 4.72 (+5.41 pct) >> 4-groups: 5.96 (0.00 pct) 4.87 (+18.28 pct) >> 8-groups: 6.58 (0.00 pct) 5.44 (+17.32 pct) >> 16-groups: 11.48 (0.00 pct) 8.07 (+29.70 pct) > > I'm really confused, so you're saying that having a process wide > spinlock is better than what this patch does? Or are you testing against > something without mm-cid entirely?
It does look like the lock contention introduced by the original mm_cid patch helped hackbench in this case. In that case, I see hackbench threads run for longer on average (avg_atom) and total idle entries are down significantly. Even on disabling C1 and C2, I see similar behavior. With the new mm_cid patch that gets rid of the lock contention, we see a drop in the hackbench performance.
I will go dig into this further meanwhile if you have any pointers please do let me know.
-- Thanks and Regards, Swapnil
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