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

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 28 Feb 2022 08:57:49 +0100 Michal Hocko <mhocko@suse.com> wrote:

> 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. 

But this cond_resched() is different from most.  This one is attempting
to yield the CPU so this task can make progress.  And cond_resched()
simply isn't an appropriate way of doing this because under this fairly
common situation, it's a no-op.