Re: [PATCH v3 2/4] mm/vmap: preload a CPU with one object for split purpose

[Uladzislau Rezki <urezki@gmail.com>]

On Mon, Jun 03, 2019 at 07:53:12PM +0200, Uladzislau Rezki wrote:
> Hello, Roman!
> 
> On Wed, May 29, 2019 at 04:34:40PM +0000, Roman Gushchin wrote:
> > On Wed, May 29, 2019 at 04:27:15PM +0200, Uladzislau Rezki wrote:
> > > Hello, Roman!
> > > 
> > > > On Mon, May 27, 2019 at 11:38:40AM +0200, Uladzislau Rezki (Sony) wrote:
> > > > > Refactor the NE_FIT_TYPE split case when it comes to an
> > > > > allocation of one extra object. We need it in order to
> > > > > build a remaining space.
> > > > > 
> > > > > Introduce ne_fit_preload()/ne_fit_preload_end() functions
> > > > > for preloading one extra vmap_area object to ensure that
> > > > > we have it available when fit type is NE_FIT_TYPE.
> > > > > 
> > > > > The preload is done per CPU in non-atomic context thus with
> > > > > GFP_KERNEL allocation masks. More permissive parameters can
> > > > > be beneficial for systems which are suffer from high memory
> > > > > pressure or low memory condition.
> > > > > 
> > > > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > > > > ---
> > > > >  mm/vmalloc.c | 79 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++---
> > > > >  1 file changed, 76 insertions(+), 3 deletions(-)
> > > > 
> > > > Hi Uladzislau!
> > > > 
> > > > This patch generally looks good to me (see some nits below),
> > > > but it would be really great to add some motivation, e.g. numbers.
> > > > 
> > > The main goal of this patch to get rid of using GFP_NOWAIT since it is
> > > more restricted due to allocation from atomic context. IMHO, if we can
> > > avoid of using it that is a right way to go.
> > > 
> > > From the other hand, as i mentioned before i have not seen any issues
> > > with that on all my test systems during big rework. But it could be
> > > beneficial for tiny systems where we do not have any swap and are
> > > limited in memory size.
> > 
> > Ok, that makes sense to me. Is it possible to emulate such a tiny system
> > on kvm and measure the benefits? Again, not a strong opinion here,
> > but it will be easier to justify adding a good chunk of code.
> > 
> It seems it is not so straightforward as it looks like. I tried it before,
> but usually the systems gets panic due to out of memory or just invokes
> the OOM killer.
> 
> I will upload a new version of it, where i embed "preloading" logic directly
> into alloc_vmap_area() function.
> 
just managed to simulate the faulty behavior of GFP_NOWAIT restriction,
resulting to failure of vmalloc allocation. Under heavy load and low
memory condition and without swap, i can trigger below warning on my
KVM machine:

<snip>
[  366.910037] Out of memory: Killed process 470 (bash) total-vm:21012kB, anon-rss:1700kB, file-rss:264kB, shmem-rss:0kB
[  366.910692] oom_reaper: reaped process 470 (bash), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB
[  367.913199] stress-ng-fork: page allocation failure: order:0, mode:0x40800(GFP_NOWAIT|__GFP_COMP), nodemask=(null),cpuset=/,mems_allowed=0
[  367.913206] CPU: 3 PID: 19951 Comm: stress-ng-fork Not tainted 5.2.0-rc3+ #999
[  367.913207] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[  367.913208] Call Trace:
[  367.913215]  dump_stack+0x5c/0x7b
[  367.913219]  warn_alloc+0x108/0x190
[  367.913222]  __alloc_pages_slowpath+0xdc7/0xdf0
[  367.913226]  __alloc_pages_nodemask+0x2de/0x330
[  367.913230]  cache_grow_begin+0x77/0x420
[  367.913232]  fallback_alloc+0x161/0x200
[  367.913235]  kmem_cache_alloc+0x1c9/0x570
[  367.913237]  alloc_vmap_area+0x98b/0xa20
[  367.913240]  __get_vm_area_node+0xb0/0x170
[  367.913243]  __vmalloc_node_range+0x6d/0x230
[  367.913246]  ? _do_fork+0xce/0x3d0
[  367.913248]  copy_process.part.46+0x850/0x1b90
[  367.913250]  ? _do_fork+0xce/0x3d0
[  367.913254]  _do_fork+0xce/0x3d0
[  367.913257]  ? __do_page_fault+0x2bf/0x4e0
[  367.913260]  do_syscall_64+0x55/0x130
[  367.913263]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  367.913265] RIP: 0033:0x7f2a8248d38b
[  367.913268] Code: db 45 85 f6 0f 85 95 01 00 00 64 4c 8b 04 25 10 00 00 00 31 d2 4d 8d 90 d0 02 00 00 31 f6 bf 11 00 20 01 b8 38 00 00 00 0f 05 <48> 3d 00 f0 ff ff 0f 87 de 00 00 00 85 c0 41 89 c5 0f 85 e5 00 00
[  367.913269] RSP: 002b:00007fff1b058c30 EFLAGS: 00000246 ORIG_RAX: 0000000000000038
[  367.913271] RAX: ffffffffffffffda RBX: 00007fff1b058c30 RCX: 00007f2a8248d38b
[  367.913272] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000001200011
[  367.913273] RBP: 00007fff1b058c80 R08: 00007f2a83d34300 R09: 00007fff1b1890a0
[  367.913274] R10: 00007f2a83d345d0 R11: 0000000000000246 R12: 0000000000000000
[  367.913275] R13: 0000000000000020 R14: 0000000000000000 R15: 0000000000000000
[  367.913278] Mem-Info:
[  367.913282] active_anon:45795 inactive_anon:80706 isolated_anon:0
                active_file:394 inactive_file:359 isolated_file:210
                unevictable:2 dirty:0 writeback:0 unstable:0
                slab_reclaimable:2691 slab_unreclaimable:21864
                mapped:80835 shmem:80740 pagetables:50422 bounce:0
                free:12185 free_pcp:776 free_cma:0
[  367.913286] Node 0 active_anon:183180kB inactive_anon:322824kB active_file:1576kB inactive_file:1436kB unevictable:8kB isolated(anon):0kB isolated(file):840kB mapped:323340kB dirty:0kB writeback:0kB shmem:322960kB shmem_thp: 0kB shmem_pmdmapped: 0kB anon_thp: 0kB writeback_tmp:0kB unstable:0kB all_unreclaimable? no
[  367.913287] Node 0 DMA free:4516kB min:724kB low:904kB high:1084kB active_anon:2384kB inactive_anon:0kB active_file:48kB inactive_file:0kB unevictable:0kB writepending:0kB present:15992kB managed:15908kB mlocked:0kB kernel_stack:1256kB pagetables:4516kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:0kB
[  367.913292] lowmem_reserve[]: 0 948 948 948
[  367.913294] Node 0 DMA32 free:44224kB min:44328kB low:55408kB high:66488kB active_anon:180252kB inactive_anon:322824kB active_file:992kB inactive_file:1332kB unevictable:8kB writepending:252kB present:1032064kB managed:995428kB mlocked:8kB kernel_stack:43260kB pagetables:197172kB bounce:0kB free_pcp:3252kB local_pcp:480kB free_cma:0kB
[  367.913299] lowmem_reserve[]: 0 0 0 0
[  367.913301] Node 0 DMA: 46*4kB (UM) 45*8kB (UM) 12*16kB (UM) 9*32kB (UM) 2*64kB (M) 2*128kB (UM) 2*256kB (M) 3*512kB (M) 1*1024kB (M) 0*2048kB 0*4096kB = 4480kB
[  367.913310] Node 0 DMA32: 966*4kB (UE) 552*8kB (UME) 648*16kB (UME) 265*32kB (UME) 75*64kB (UME) 12*128kB (ME) 1*256kB (U) 1*512kB (E) 1*1024kB (U) 2*2048kB (UM) 1*4096kB (M) = 43448kB
[  367.913322] Node 0 hugepages_total=0 hugepages_free=0 hugepages_surp=0 hugepages_size=2048kB
[  367.913323] 81750 total pagecache pages
[  367.913324] 0 pages in swap cache
[  367.913325] Swap cache stats: add 0, delete 0, find 0/0
[  367.913325] Free swap  = 0kB
[  367.913326] Total swap = 0kB
[  367.913327] 262014 pages RAM
[  367.913327] 0 pages HighMem/MovableOnly
[  367.913328] 9180 pages reserved
[  367.913329] 0 pages hwpoisoned
[  372.338733] systemd-journald[195]: /dev/kmsg buffer overrun, some messages lost.
<snip>

Whereas with "preload" logic i see only OOM killer related messages:

<snip>
[  136.787266] oom-kill:constraint=CONSTRAINT_NONE,nodemask=(null),cpuset=/,mems_allowed=0,global_oom,task_memcg=/,task=systemd-journal,pid=196,uid=0
[  136.787276] Out of memory: Killed process 196 (systemd-journal) total-vm:56832kB, anon-rss:512kB, file-rss:336kB, shmem-rss:820kB
[  136.790481] oom_reaper: reaped process 196 (systemd-journal), now anon-rss:0kB, file-rss:0kB, shmem-rss:820kB
<snip>

i.e. vmalloc still able to allocate.

Probably i need to update the commit message by this simulation and finding.

--
Vlad Rezki