On Thu, Aug 29, 2024 at 09:06:33PM +0800, Adrian Huang wrote: > From: Adrian Huang <ahuang12@xxxxxxxxxx> > > 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 > > Signed-off-by: Adrian Huang <ahuang12@xxxxxxxxxx> > --- > mm/vmalloc.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c > index 3f9b6bd707d2..607697c81e60 100644 > --- a/mm/vmalloc.c > +++ b/mm/vmalloc.c > @@ -2210,6 +2210,7 @@ static void purge_vmap_node(struct work_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); > > @@ -2224,7 +2225,7 @@ static void purge_vmap_node(struct work_struct *work) > > list_del_init(&va->list); > > - atomic_long_sub(nr, &vmap_lazy_nr); > + nr_purged_pages += nr; > vn->nr_purged++; > > if (is_vn_id_valid(vn_id) && !vn->skip_populate) > @@ -2235,6 +2236,8 @@ static void purge_vmap_node(struct work_struct *work) > list_add(&va->list, &local_list); > } > > + atomic_long_sub(nr_purged_pages, &vmap_lazy_nr); > + > reclaim_list_global(&local_list); > } > > -- > 2.34.1 > I see the point and it looks good to me. Reviewed-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx> Thank you for improving this. There is one more spot which i detected earlier, it is: <snip> static void free_vmap_area_noflush(struct vmap_area *va) { unsigned long nr_lazy_max = lazy_max_pages(); unsigned long va_start = va->va_start; unsigned int vn_id = decode_vn_id(va->flags); struct vmap_node *vn; unsigned long nr_lazy; if (WARN_ON_ONCE(!list_empty(&va->list))) return; nr_lazy = atomic_long_add_return((va->va_end - va->va_start) >> PAGE_SHIFT, &vmap_lazy_nr); ... <snip> atomic_long_add_return() might also introduce a high contention. We can optimize by splitting into more light atomics. Can you check it on your 448-cores system? Tnanks! -- Uladzislau Rezki
