Messages in this thread | | | Date | Tue, 19 Sep 2023 12:00:01 +0530 | Subject | Re: [RFC PATCH V1 0/6] sched/numa: Enhance disjoint VMA scanning | From | Raghavendra K T <> |
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On 8/29/2023 11:36 AM, Raghavendra K T wrote: > Since commit fc137c0ddab2 ("sched/numa: enhance vma scanning logic") [1] > VMA scanning is allowed if: > 1) The task had accessed the VMA. > Rationale: Reduce overhead for the tasks that had not > touched VMA. Also filter out unnecessary scanning. > > 2) Early phase of the VMA scan where mm->numa_scan_seq is less than 2. > Rationale: Understanding initial characteristics of VMAs and also > prevent VMA scanning unfairness. > > While that works for most of the times to reduce scanning overhead, > there are some corner cases associated with it. > > This was found in an internal LKP run and also reported by [2]. There was > an attempt to fix. > > Link: https://lore.kernel.org/linux-mm/cover.1685506205.git.raghavendra.kt@amd.com/T/ > > This is a fully different series after Mel's feedback to address the issue > and also a continuation of enhancing VMA scanning for NUMA balancing. > > Problem statement (Disjoint VMA set): > ====================================== > Let's look at some of the corner cases with a below example of tasks and their > access pattern. > > Consider N tasks (threads) of a process. > Set1 tasks accessing vma_x (group of VMAs) > Set2 tasks accessing vma_y (group of VMAs) > > Set1 Set2 > ------------------- -------------------- > | task_1..task_n/2 | | task_n/2+1..task_n | > ------------------- -------------------- > | | > V V > ------------------- -------------------- > | vma_x | | vma_y | > ------------------- -------------------- > > Corner cases: > (a) Out of N tasks, not all of them gets fair opportunity to scan. (PeterZ). > suppose Set1 tasks gets more opportunity to scan (May be because of the > activity pattern of tasks or other reasons in current design) in the above > example, then vma_x gets scanned more number of times than vma_y. > > some experiment is also done here which illustrates this unfairness: > Link: https://lore.kernel.org/lkml/c730dee0-a711-8a8e-3eb1-1bfdd21e6add@amd.com/ > > (b) Sizes of vmas can differ. > Suppose size of vma_y is far greater than the size of vma_x, then a bigger > portion of vma_y can potentially be left unscanned since scanning is bounded > by scan_size of 256MB (default) for each iteration. > > (c) Highly active threads trap a few VMAs frequently, and some of the VMAs not > accessed for long time can potentially get starved of scanning indefinitely > (Mel). There is a possibility of lack of enough hints/details about VMAs if it > is needed later for migration. > > (d) Allocation of memory in some specific manner (Mel). > One example could be, Suppose a main thread allocates memory and it is not > active. When other threads tries to act upon it, they may not have much > hints about it, if the corresponding VMA was not scanned. > > (e) VMAs that are created after two full scans of mm (mm->numa_scan_seq > 2) > will never get scanned. (Observed rarely but very much possible depending on > workload behaviour). > > Above this, a combination of some of the above (e.g., (a) and (b)) can > potentially amplifyi/worsen the side effect. > > This patchset, tries to address the above issues by enhancing unconditional > VMA scanning logic. > > High level ideas: > ================= > Idea-1) Depending on vma_size, populate a per vma_scan_select value, decrement it > and when it hits zero do force scan (Mel). > vma_scan_select value is again repopulated when it hits zero. > > This is how VMA scanning phases looks like after implementation: > > |<---p1--->|<-----p2----->|<-----p2----->|... > > Algorithm: > p1: New VMA, initial phase do not scan till scan_delay. > > p2: Allow scanning if the task has accessed VMA or vma_scan_select hit zero. > > Reinitialize vma_scan_select and repeat p2. > > pros/cons: > + : Ratelimiting is inbuilt to the approach > + : vma_size is taken into account for scanning > +/-: Scanning continues forever > - : Changes in vma size is taken care after force scan. i.e., > vma_scan_select is repopulated only after vma_scan_select hits zero. > > Idea-1 can potentially cover all the issues mentioned above. > > Idea-2) Take bitmask_weight of latest access_pids value (suggested by Bharata). > If number of tasks accessing vma is >= 1, unconditionally allow scanning. > > Idea-3 ) Take bitmask_weight of access_pid history of VMA. If number of tasks > accessing VMA is > THRESHOLD (=3), unconditionally allow scanning. > > Rationale (Idea-2,3): Do not miss out scanning of critical VMAs. > > Idea-4) Have a per vma_scan_seq. allow the unconditional scan till vma_scan_seq > reaches a value proportional (or equal) to vma_size/scan_size. > This a complimentary to Idea-1. > > this is how VMA scanning phases looks like after implementation: > > |<--p1--->|<-----p2----->|<-----p3----->|<-----p4----->...||<-----p2----->|<-----p3----->|<-----p4-----> ...|| > RESET RESET > Algorithm: > p1: New VMA, initial phase do not scan till scan_delay. > > p2: Allow scanning if task has accessed VMA or vma_scan_seq has reached till > f(vma_size)/scan_size) for e.g., f = 1/2 * vma_size/scan_size. > > p3: Allow scanning if task has accessed VMA or vma_scan_seq has reached till > f(vma_size)/scan_size in a rate limited manner. This is an optional phase. > > p4: Allow scanning iff task has accessed VMA. > > Reset after p4 (optional). > > Repeat p2, p3 p4 > > Motivation: Allow agressive scanning in the beginning followed by a rate > limited scanning. And then completely disallow scanning to avoid unnecessary > scanning. Reset time could be a function of scan_delay and chosen long enough > to aid long running task to forget history and start afresh. > > + : Ratelimiting need to be taken care separately if needed. > +/-: Scanning continues only if RESET of vma_scan_seq is implemented. > + : changes in vma size is taken care in every scan. > > Current patch series implements Ideas 1, 2, 3 + extension of access PID history > idea from PeterZ. > > Results: > ====== > Base: 6.5.0-rc6+ (4853c74bd7ab) > SUT: Milan w/ 2 numa nodes 256 cpus > > mmtest numa01_THREAD_ALLOC manual run: > > base patched > real 1m22.758s 1m9.200s > user 249m49.540s 229m30.039s > sys 0m25.040s 3m10.451s > > numa_pte_updates 6985 1573363 > numa_hint_faults 2705 1022623 > numa_hint_faults_local 2279 389633 > numa_pages_migrated 426 632990 > > kernbench > base patched > Amean user-256 21989.09 ( 0.00%) 21677.36 * 1.42%* > Amean syst-256 10171.34 ( 0.00%) 10818.28 * -6.36%* > Amean elsp-256 166.81 ( 0.00%) 168.40 * -0.95%* > > Duration User 65973.18 65038.00 > Duration System 30538.92 32478.59 > Duration Elapsed 529.52 533.09 > > Ops NUMA PTE updates 976844.00 962680.00 > Ops NUMA hint faults 226763.00 245620.00 > Ops NUMA pages migrated 220146.00 207025.00 > Ops AutoNUMA cost 1144.84 1238.77 > > Improvements in other benchmarks I have tested. > Time based: > Hashjoin 4.21% > Btree 2.04% > XSbench 0.36% > > Throughput based: > Graph500 -3.62% > Nas.bt 3.69% > Nas.ft 21.91% > > Note: VMA scanning improvements [1] has refined scanning so much that > system overhead we re-introduce with additional scan look glaringly > high. But If we consider the difference between before [1] and current > series, overall scanning overhead is considerably reduced. > > 1. Link: https://lore.kernel.org/lkml/cover.1677672277.git.raghavendra.kt@amd.com/T/#t > 2. Link: https://lore.kernel.org/lkml/cover.1683033105.git.raghavendra.kt@amd.com/ > > Note: Patch description is again repeated in some patches to avoid any > need to copy from cover letter again. > > Peter Zijlstra (1): > sched/numa: Increase tasks' access history > > Raghavendra K T (5): > sched/numa: Move up the access pid reset logic > sched/numa: Add disjoint vma unconditional scan logic > sched/numa: Remove unconditional scan logic using mm numa_scan_seq > sched/numa: Allow recently accessed VMAs to be scanned > sched/numa: Allow scanning of shared VMAs > > include/linux/mm.h | 12 +++-- > include/linux/mm_types.h | 5 +- > kernel/sched/fair.c | 109 ++++++++++++++++++++++++++++++++------- > 3 files changed, 102 insertions(+), 24 deletions(-) >
Hello Andrew,
I am Resending patch rebasing to mm-unstable, adding results from Oliver and Swapnil.
(so that it is ready to merge once we get go ahead/ no objection from Mel, Peter ... Okay to work any further suggestions if any).
Hope that is Okay.
Thanks and Regards - Raghu
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