Re: [PATCH linux-next] mm: swap: get rid of deadloop in swapin readahead

[Michal Hocko <mhocko@suse.com>]

On Mon 21-02-22 11:17:49, cgel.zte@gmail.com wrote:
> From: Guo Ziliang <guo.ziliang@zte.com.cn>
> 
> In our testing, a deadloop task was found. Through sysrq printing, same 
> stack was found every time, as follows:
> __swap_duplicate+0x58/0x1a0
> swapcache_prepare+0x24/0x30
> __read_swap_cache_async+0xac/0x220
> read_swap_cache_async+0x58/0xa0
> swapin_readahead+0x24c/0x628
> do_swap_page+0x374/0x8a0
> __handle_mm_fault+0x598/0xd60
> handle_mm_fault+0x114/0x200
> do_page_fault+0x148/0x4d0
> do_translation_fault+0xb0/0xd4
> do_mem_abort+0x50/0xb0
> 
> The reason for the deadloop is that swapcache_prepare() always returns
> EEXIST, indicating that SWAP_HAS_CACHE has not been cleared, so that
> it cannot jump out of the loop. We suspect that the task that clears
> the SWAP_HAS_CACHE flag never gets a chance to run. We try to lower
> the priority of the task stuck in a deadloop so that the task that
> clears the SWAP_HAS_CACHE flag will run. The results show that the
> system returns to normal after the priority is lowered.
> 
> In our testing, multiple real-time tasks are bound to the same core,
> and the task in the deadloop is the highest priority task of the
> core, so the deadloop task cannot be preempted.
> 
> Although cond_resched() is used by __read_swap_cache_async, it is an
> empty function in the preemptive system and cannot achieve the purpose
> of releasing the CPU. A high-priority task cannot release the CPU
> unless preempted by a higher-priority task. But when this task
> is already the highest priority task on this core, other tasks
> will not be able to be scheduled. So we think we should replace
> cond_resched() with schedule_timeout_uninterruptible(1),
> schedule_timeout_interruptible will call set_current_state
> first to set the task state, so the task will be removed
> from the running queue, so as to achieve the purpose of
> giving up the CPU and prevent it from running in kernel
> mode for too long.

I am sorry but I really do not see how this case is any different from
any other kernel code path being hogged by a RT task. We surely
shouldn't put sleeps into all random paths which are doing cond_resched
at the moment. 

> Reported-by: Zeal Robot <zealci@zte.com.cn>
> Reviewed-by: Ran Xiaokai <ran.xiaokai@zte.com.cn>
> Reviewed-by: Jiang Xuexin <jiang.xuexin@zte.com.cn>
> Reviewed-by: Yang Yang <yang.yang29@zte.com.cn>
> Signed-off-by: Guo Ziliang <guo.ziliang@zte.com.cn>
> ---
>  mm/swap_state.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/swap_state.c b/mm/swap_state.c
> index 8d4104242100..ee67164531c0 100644
> --- a/mm/swap_state.c
> +++ b/mm/swap_state.c
> @@ -478,7 +478,7 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
>  		 * __read_swap_cache_async(), which has set SWAP_HAS_CACHE
>  		 * in swap_map, but not yet added its page to swap cache.
>  		 */
> -		cond_resched();
> +		schedule_timeout_uninterruptible(1);
>  	}
>  
>  	/*
> -- 
> 2.15.2
> 

-- 
Michal Hocko
SUSE Labs