The patch titled Subject: mm: vmalloc: optimize vmap_lazy_nr arithmetic when purging each vmap_area has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-vmalloc-optimize-vmap_lazy_nr-arithmetic-when-purging-each-vmap_area.patch
This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...
This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------ From: Adrian Huang ahuang12@lenovo.com Subject: mm: vmalloc: optimize vmap_lazy_nr arithmetic when purging each vmap_area Date: Thu, 29 Aug 2024 21:06:33 +0800
When running the vmalloc stress on a 448-core system, observe the average latency of purge_vmap_node() is about 2 seconds by using the eBPF/bcc 'funclatency.py' tool [1].
# /your-git-repo/bcc/tools/funclatency.py -u purge_vmap_node & pid1=$! && sleep 8 && modprobe test_vmalloc nr_threads=$(nproc) run_test_mask=0x7; kill -SIGINT $pid1
usecs : count distribution 0 -> 1 : 0 | | 2 -> 3 : 29 | | 4 -> 7 : 19 | | 8 -> 15 : 56 | | 16 -> 31 : 483 |**** | 32 -> 63 : 1548 |************ | 64 -> 127 : 2634 |********************* | 128 -> 255 : 2535 |********************* | 256 -> 511 : 1776 |************** | 512 -> 1023 : 1015 |******** | 1024 -> 2047 : 573 |**** | 2048 -> 4095 : 488 |**** | 4096 -> 8191 : 1091 |********* | 8192 -> 16383 : 3078 |************************* | 16384 -> 32767 : 4821 |****************************************| 32768 -> 65535 : 3318 |*************************** | 65536 -> 131071 : 1718 |************** | 131072 -> 262143 : 2220 |****************** | 262144 -> 524287 : 1147 |********* | 524288 -> 1048575 : 1179 |********* | 1048576 -> 2097151 : 822 |****** | 2097152 -> 4194303 : 906 |******* | 4194304 -> 8388607 : 2148 |***************** | 8388608 -> 16777215 : 4497 |************************************* | 16777216 -> 33554431 : 289 |** |
avg = 2041714 usecs, total: 78381401772 usecs, count: 38390
The worst case is over 16-33 seconds, so soft lockup is triggered [2].
[Root Cause] 1) Each purge_list has the long list. The following shows the number of vmap_area is purged.
crash> p vmap_nodes vmap_nodes = $27 = (struct vmap_node *) 0xff2de5a900100000 crash> vmap_node 0xff2de5a900100000 128 | grep nr_purged nr_purged = 663070 ... nr_purged = 821670 nr_purged = 692214 nr_purged = 726808 ...
2) atomic_long_sub() employs the 'lock' prefix to ensure the atomic operation when purging each vmap_area. However, the iteration is over 600000 vmap_area (See 'nr_purged' above).
Here is objdump output:
$ objdump -D vmlinux ffffffff813e8c80 <purge_vmap_node>: ... ffffffff813e8d70: f0 48 29 2d 68 0c bb lock sub %rbp,0x2bb0c68(%rip) ...
Quote from "Instruction tables" pdf file [3]: Instructions with a LOCK prefix have a long latency that depends on cache organization and possibly RAM speed. If there are multiple processors or cores or direct memory access (DMA) devices, then all locked instructions will lock a cache line for exclusive access, which may involve RAM access. A LOCK prefix typically costs more than a hundred clock cycles, even on single-processor systems.
That's why the latency of purge_vmap_node() dramatically increases on a many-core system: One core is busy on purging each vmap_area of the *long* purge_list and executing atomic_long_sub() for each vmap_area, while other cores free vmalloc allocations and execute atomic_long_add_return() in free_vmap_area_noflush().
[Solution] Employ a local variable to record the total purged pages, and execute atomic_long_sub() after the traversal of the purge_list is done. The experiment result shows the latency improvement is 99%.
[Experiment Result] 1) System Configuration: Three servers (with HT-enabled) are tested. * 72-core server: 3rd Gen Intel Xeon Scalable Processor*1 * 192-core server: 5th Gen Intel Xeon Scalable Processor*2 * 448-core server: AMD Zen 4 Processor*2
2) Kernel Config * CONFIG_KASAN is disabled
3) The data in column "w/o patch" and "w/ patch" * Unit: micro seconds (us) * Each data is the average of 3-time measurements
System w/o patch (us) w/ patch (us) Improvement (%) --------------- -------------- ------------- ------------- 72-core server 2194 14 99.36% 192-core server 143799 1139 99.21% 448-core server 1992122 6883 99.65%
[1] https://github.com/iovisor/bcc/blob/master/tools/funclatency.py [2] https://gist.github.com/AdrianHuang/37c15f67b45407b83c2d32f918656c12 [3] https://www.agner.org/optimize/instruction_tables.pdf
Link: https://lkml.kernel.org/r/20240829130633.2184-1-ahuang12@lenovo.com Signed-off-by: Adrian Huang ahuang12@lenovo.com Reviewed-by: Uladzislau Rezki (Sony) urezki@gmail.com Cc: Christoph Hellwig hch@infradead.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
mm/vmalloc.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
--- a/mm/vmalloc.c~mm-vmalloc-optimize-vmap_lazy_nr-arithmetic-when-purging-each-vmap_area +++ a/mm/vmalloc.c @@ -2191,6 +2191,7 @@ static void purge_vmap_node(struct work_ { struct vmap_node *vn = container_of(work, struct vmap_node, purge_work); + unsigned long nr_purged_pages = 0; struct vmap_area *va, *n_va; LIST_HEAD(local_list);
@@ -2208,7 +2209,7 @@ static void purge_vmap_node(struct work_ kasan_release_vmalloc(orig_start, orig_end, va->va_start, va->va_end);
- atomic_long_sub(nr, &vmap_lazy_nr); + nr_purged_pages += nr; vn->nr_purged++;
if (is_vn_id_valid(vn_id) && !vn->skip_populate) @@ -2219,6 +2220,8 @@ static void purge_vmap_node(struct work_ list_add(&va->list, &local_list); }
+ atomic_long_sub(nr_purged_pages, &vmap_lazy_nr); + reclaim_list_global(&local_list); }
_
Patches currently in -mm which might be from ahuang12@lenovo.com are
mm-vmalloc-optimize-vmap_lazy_nr-arithmetic-when-purging-each-vmap_area.patch mm-vmalloc-combine-all-tlb-flush-operations-of-kasan-shadow-virtual-address-into-one-operation.patch