Re: [PATCH V3] mm/mprotect: Add a cond_resched() inside change_pmd_range()

[Michal Hocko]

On Thu 14-12-17 19:35:51, Anshuman Khandual wrote:
> While testing on a large CPU system, detected the following RCU
> stall many times over the span of the workload. This problem
> is solved by adding a cond_resched() in the change_pmd_range()
> function.
> 
> [  850.962530] INFO: rcu_sched detected stalls on CPUs/tasks:
> [  850.962584]  154-....: (670 ticks this GP) idle=022/140000000000000/0 softirq=2825/2825 fqs=612
> [  850.962605]  (detected by 955, t=6002 jiffies, g=4486, c=4485, q=90864)
> [  850.962895] Sending NMI from CPU 955 to CPUs 154:
> [  850.992667] NMI backtrace for cpu 154
> [  850.993069] CPU: 154 PID: 147071 Comm: workload Not tainted 4.15.0-rc3+ #3
> [  850.993258] NIP:  c0000000000b3f64 LR: c0000000000b33d4 CTR: 000000000000aa18
> [  850.993503] REGS: 00000000a4b0fb44 TRAP: 0501   Not tainted  (4.15.0-rc3+)
> [  850.993707] MSR:  8000000000009033 <SF,EE,ME,IR,DR,RI,LE>  CR: 22422082  XER: 00000000
> [  850.994386] CFAR: 00000000006cf8f0 SOFTE: 1
> GPR00: 0010000000000000 c00003ef9b1cb8c0 c0000000010cc600 0000000000000000
> GPR04: 8e0000018c32b200 40017b3858fd6e00 8e0000018c32b208 40017b3858fd6e00
> GPR08: 8e0000018c32b210 40017b3858fd6e00 8e0000018c32b218 40017b3858fd6e00
> GPR12: ffffffffffffffff c00000000fb25100
> [  850.995976] NIP [c0000000000b3f64] plpar_hcall9+0x44/0x7c
> [  850.996174] LR [c0000000000b33d4] pSeries_lpar_flush_hash_range+0x384/0x420
> [  850.996401] Call Trace:
> [  850.996600] [c00003ef9b1cb8c0] [c00003fa8fff7d40] 0xc00003fa8fff7d40 (unreliable)
> [  850.996959] [c00003ef9b1cba40] [c0000000000688a8] flush_hash_range+0x48/0x100
> [  850.997261] [c00003ef9b1cba90] [c000000000071b14] __flush_tlb_pending+0x44/0xd0
> [  850.997600] [c00003ef9b1cbac0] [c000000000071fa8] hpte_need_flush+0x408/0x470
> [  850.997958] [c00003ef9b1cbb30] [c0000000002c646c] change_protection_range+0xaac/0xf10
> [  850.998180] [c00003ef9b1cbcb0] [c0000000002f2510] change_prot_numa+0x30/0xb0
> [  850.998502] [c00003ef9b1cbce0] [c00000000013a950] task_numa_work+0x2d0/0x3e0
> [  850.998816] [c00003ef9b1cbda0] [c00000000011ea30] task_work_run+0x130/0x190
> [  850.999121] [c00003ef9b1cbe00] [c00000000001bcd8] do_notify_resume+0x118/0x120
> [  850.999421] [c00003ef9b1cbe30] [c00000000000b744] ret_from_except_lite+0x70/0x74
> [  850.999716] Instruction dump:
> [  850.999959] 60000000 f8810028 7ca42b78 7cc53378 7ce63b78 7d074378 7d284b78 7d495378
> [  851.000575] e9410060 e9610068 e9810070 44000022 <7d806378> e9810028 f88c0000 f8ac0008
> 
> Suggested-by: Nicholas Piggin <npiggin@xxxxxxxxx>
> Signed-off-by: Anshuman Khandual <khandual@xxxxxxxxxxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks!

> ---
> Changes in V3:
> 
> - Enabled the scheduling point for THP backed pages and pmd holes
> 
> Changes in V2: (https://patchwork.kernel.org/patch/10111863/)
> 
> - Moved cond_resched() to change_pmd_range() as per Michal Hocko
> - Fixed commit message as appropriate
> 
> Changes in V1: (https://patchwork.kernel.org/patch/10111445/)
> 
>  mm/mprotect.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index ec39f73..58b629b 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -166,7 +166,7 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
>  		next = pmd_addr_end(addr, end);
>  		if (!is_swap_pmd(*pmd) && !pmd_trans_huge(*pmd) && !pmd_devmap(*pmd)
>  				&& pmd_none_or_clear_bad(pmd))
> -			continue;
> +			goto next;
>  
>  		/* invoke the mmu notifier if the pmd is populated */
>  		if (!mni_start) {
> @@ -188,7 +188,7 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
>  					}
>  
>  					/* huge pmd was handled */
> -					continue;
> +					goto next;
>  				}
>  			}
>  			/* fall through, the trans huge pmd just split */
> @@ -196,6 +196,8 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
>  		this_pages = change_pte_range(vma, pmd, addr, next, newprot,
>  				 dirty_accountable, prot_numa);
>  		pages += this_pages;
> +next:
> +		cond_resched();
>  	} while (pmd++, addr = next, addr != end);
>  
>  	if (mni_start)
> -- 
> 2.9.3
> 

-- 
Michal Hocko
SUSE Labs