Re: LTP hugemmap05 test case failure on arm64 with linux-next (next-20190613)

[Mike Kravetz <mike.kravetz@oracle.com>]

On 6/24/19 2:30 PM, Qian Cai wrote:
> So the problem is that ipcget_public() has held the semaphore "ids->rwsem" for
> too long seems unnecessarily and then goes to sleep sometimes due to direct
> reclaim (other times LTP hugemmap05 [1] has hugetlb_file_setup() returns
> -ENOMEM),

Thanks for looking into this!  I noticed that recent kernels could take a
VERY long time trying to do high order allocations.  In my case it was trying
to do dynamic hugetlb page allocations as well [1].  But, IMO this is more
of a general direct reclaim/compation issue than something hugetlb specific.

> 
> [  788.765739][ T1315] INFO: task hugemmap05:5001 can't die for more than 122
> seconds.
> [  788.773512][ T1315] hugemmap05      R  running task    25600  5001      1
> 0x0000000d
> [  788.781348][ T1315] Call trace:
> [  788.784536][ T1315]  __switch_to+0x2e0/0x37c
> [  788.788848][ T1315]  try_to_free_pages+0x614/0x934
> [  788.793679][ T1315]  __alloc_pages_nodemask+0xe88/0x1d60
> [  788.799030][ T1315]  alloc_fresh_huge_page+0x16c/0x588
> [  788.804206][ T1315]  alloc_surplus_huge_page+0x9c/0x278
> [  788.809468][ T1315]  hugetlb_acct_memory+0x114/0x5c4
> [  788.814469][ T1315]  hugetlb_reserve_pages+0x170/0x2b0
> [  788.819662][ T1315]  hugetlb_file_setup+0x26c/0x3a8
> [  788.824600][ T1315]  newseg+0x220/0x63c
> [  788.828490][ T1315]  ipcget+0x570/0x674
> [  788.832377][ T1315]  ksys_shmget+0x90/0xc4
> [  788.836525][ T1315]  __arm64_sys_shmget+0x54/0x88
> [  788.841282][ T1315]  el0_svc_handler+0x19c/0x26c
> [  788.845952][ T1315]  el0_svc+0x8/0xc
> 
> and then all other processes are waiting on the semaphore causes lock
> contentions,

That call to hugetlb_file_setup() via ipcget certainly could take a long
time to execute.  In the default case huge pages are reserved to back the
shared memory segment.  If these pages were not prealllocated, then the
code will try to dynamically allocate the required number of huge pages.
So, even if [1] were not an issue I think a change here makes sense.

> [  788.849583][ T1315] INFO: task hugemmap05:5027 blocked for more than 122
> seconds.
> [  788.857119][ T1315]       Tainted: G        W         5.2.0-rc6-next-20190624 
> #2
> [  788.864566][ T1315] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
> disables this message.
> [  788.873139][ T1315] hugemmap05      D26960  5027   5026 0x00000000
> [  788.879395][ T1315] Call trace:
> [  788.882576][ T1315]  __switch_to+0x2e0/0x37c
> [  788.886901][ T1315]  __schedule+0xb74/0xf0c
> [  788.891136][ T1315]  schedule+0x60/0x168
> [  788.895097][ T1315]  rwsem_down_write_slowpath+0x5a0/0x8c8
> [  788.900653][ T1315]  down_write+0xc0/0xc4
> [  788.904715][ T1315]  ipcget+0x74/0x674
> [  788.908516][ T1315]  ksys_shmget+0x90/0xc4
> [  788.912664][ T1315]  __arm64_sys_shmget+0x54/0x88
> [  788.917420][ T1315]  el0_svc_handler+0x19c/0x26c
> [  788.922088][ T1315]  el0_svc+0x8/0xc
> 
> Ideally, it seems only ipc_findkey() and newseg() in this path needs to hold the
> semaphore to protect concurrency access, so it could just be converted to a
> spinlock instead.

I do not have enough experience with this ipc code to comment on your proposed
change.  But, I will look into it.

[1] https://lkml.org/lkml/2019/4/23/2
-- 
Mike Kravetz

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