On Mon 28-02-22 07:33:15, Andrew Morton wrote: > On Mon, 28 Feb 2022 08:57:49 +0100 Michal Hocko <mhocko@xxxxxxxx> wrote: > > > On Mon 21-02-22 11:17:49, cgel.zte@xxxxxxxxx wrote: > > > From: Guo Ziliang <guo.ziliang@xxxxxxxxxx> > > > > > > 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. I might be really missing something but I really do not see how is this any different from the page allocator path which only does cond_resched as well (well, except for throttling but that might just not trigger). Or other paths which just do cond_resched while waiting for a progress somewhere else. Not that I like this situation but !PREEMPT kernel with RT priority tasks is rather limited and full of potential priblems IMHO. -- Michal Hocko SUSE Labs
