On Fri 03-03-17 19:48:30, Tetsuo Handa wrote: > Continued from http://lkml.kernel.org/r/201702261530.JDD56292.OFOLFHQtVMJSOF@xxxxxxxxxxxxxxxxxxx : > > While I was testing a patch which avoids infinite too_many_isolated() loop in > shrink_inactive_list(), I hit a lockup where WQ_MEM_RECLAIM threads got stuck > waiting for memory allocation. I guess that we overlooked a basic thing about > WQ_MEM_RECLAIM. > > WQ_MEM_RECLAIM helps only when the cause of failing to complete > a work item is lack of "struct task_struct" to run that work item, for > WQ_MEM_RECLAIM preallocates one "struct task_struct" so that the workqueue > will not be blocked waiting for memory allocation for "struct task_struct". > > WQ_MEM_RECLAIM does not help when "struct task_struct" running that work > item is blocked waiting for memory allocation (or is indirectly blocked > on a lock where the owner of that lock is blocked waiting for memory > allocation). That is, WQ_MEM_RECLAIM users must guarantee forward progress > if memory allocation (including indirect memory allocation via > locks/completions) is needed. > > In XFS, "xfs_mru_cache", "xfs-buf/%s", "xfs-data/%s", "xfs-conv/%s", "xfs-cil/%s", > "xfs-reclaim/%s", "xfs-log/%s", "xfs-eofblocks/%s", "xfsalloc" and "xfsdiscard" > workqueues are used, and all but "xfsdiscard" are WQ_MEM_RECLAIM workqueues. > > What I observed is at http://I-love.SAKURA.ne.jp/tmp/serial-20170226.txt.xz . > I guess that the key of this lockup is that xfs-data/sda1 and xfs-eofblocks/s > workqueues (which are RESCUER) got stuck waiting for memory allocation. If those workers are really required for a further progress of the memory reclaim then they shouldn't block on allocation at all and either use pre allocated memory or use PF_MEMALLOC in case there is a guarantee that only very limited amount of memory is allocated from that context and there will be at least the same amount of memory freed as a result in a reasonable time. This is something for xfs people to answer though. Please note that I didn't really have time to look through the below traces so the above note is rather generic. It would be really helpful if you could provide a high level dependency chains to see why those rescuers are necessary for the forward progress because it is really easy to get lost in so many traces. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
