On 4 Aug 2023, at 2:18, Byungchul Park wrote: > Implementation of CONFIG_MIGRC that stands for 'Migration Read Copy'. > > We always face the migration overhead at either promotion or demotion, > while working with tiered memory e.g. CXL memory and found out TLB > shootdown is a quite big one that is needed to get rid of if possible. > > Fortunately, TLB flush can be defered or even skipped if both source and > destination of folios during migration are kept until all TLB flushes > required will have been done, of course, only if the target PTE entries > have read only permission, more precisely speaking, don't have write > permission. Otherwise, no doubt the folio might get messed up. So this would only reduce or eliminate TLB flushes? The same goal should be achievable with batched TLB flush, right? You probably can group to-be-migrated pages into a read-only group and a writable group and migrate the read-only group first then the writable group. It would reduce or eliminate the TLB flushes for the read-only group of pages, right? > > To achieve that: > > 1. For the folios that have only non-writable TLB entries, prevent > TLB flush by keeping both source and destination of folios during > migration, which will be handled later at a better time. In this case, the page table points to the destination folio, but TLB would cache the old translation pointing to the source folio. I wonder if there would be any correctness issue. > > 2. When any non-writable TLB entry changes to writable e.g. through > fault handler, give up CONFIG_MIGRC mechanism so as to perform > TLB flush required right away. > > 3. TLB flushes can be skipped if all TLB flushes required to free the > duplicated folios have been done by any reason, which doesn't have > to be done from migrations. > > 4. Adjust watermark check routine, __zone_watermark_ok(), with the > number of duplicated folios because those folios can be freed > and obtained right away through appropreate TLB flushes. > > 5. Perform TLB flushes and free the duplicated folios pending the > flushes if page allocation routine is in trouble due to memory > pressure, even more aggresively for high order allocation. > > The measurement result: > > Architecture - x86_64 > QEMU - kvm enabled, host cpu, 2nodes((4cpus, 2GB)+(cpuless, 6GB)) > Linux Kernel - v6.4, numa balancing tiering on, demotion enabled > Benchmark - XSBench with no parameter changed > > run 'perf stat' using events: > (FYI, process wide result ~= system wide result(-a option)) > 1) itlb.itlb_flush > 2) tlb_flush.dtlb_thread > 3) tlb_flush.stlb_any > > run 'cat /proc/vmstat' and pick up: > 1) pgdemote_kswapd > 2) numa_pages_migrated > 3) pgmigrate_success > 4) nr_tlb_remote_flush > 5) nr_tlb_remote_flush_received > 6) nr_tlb_local_flush_all > 7) nr_tlb_local_flush_one > > BEFORE - mainline v6.4 > ========================================== > > $ perf stat -e itlb.itlb_flush,tlb_flush.dtlb_thread,tlb_flush.stlb_any ./XSBench > > Performance counter stats for './XSBench': > > 426856 itlb.itlb_flush > 6900414 tlb_flush.dtlb_thread > 7303137 tlb_flush.stlb_any > > 33.500486566 seconds time elapsed > 92.852128000 seconds user > 10.526718000 seconds sys > > $ cat /proc/vmstat > > ... > pgdemote_kswapd 1052596 > numa_pages_migrated 1052359 > pgmigrate_success 2161846 > nr_tlb_remote_flush 72370 > nr_tlb_remote_flush_received 213711 > nr_tlb_local_flush_all 3385 > nr_tlb_local_flush_one 198679 > ... > > AFTER - mainline v6.4 + CONFIG_MIGRC > ========================================== > > $ perf stat -e itlb.itlb_flush,tlb_flush.dtlb_thread,tlb_flush.stlb_any ./XSBench > > Performance counter stats for './XSBench': > > 179537 itlb.itlb_flush > 6131135 tlb_flush.dtlb_thread > 6920979 tlb_flush.stlb_any > > 30.396700625 seconds time elapsed > 80.331252000 seconds user > 10.303761000 seconds sys > > $ cat /proc/vmstat > > ... > pgdemote_kswapd 1044602 > numa_pages_migrated 1044202 > pgmigrate_success 2157808 > nr_tlb_remote_flush 30453 > nr_tlb_remote_flush_received 88840 > nr_tlb_local_flush_all 3039 > nr_tlb_local_flush_one 198875 > ... > > Signed-off-by: Byungchul Park <byungchul@xxxxxx> > --- -- Best Regards, Yan, Zi
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