On Wed, Dec 09, 2015 at 06:15:08PM +0100, Andrea Arcangeli wrote: > Hello Petr, > > On Wed, Dec 09, 2015 at 05:19:59PM +0100, Petr Holasek wrote: > > Hi Andrea, > > > > I've been running stress tests against this patchset for a couple of hours > > and everything was ok. However, I've allocated ~1TB of memory and got > > following lockup during disabling KSM with 'echo 2 > /sys/kernel/mm/ksm/run': > > > > [13201.060601] INFO: task ksmd:351 blocked for more than 120 seconds. > > [13201.066812] Not tainted 4.4.0-rc4+ #5 > > [13201.070996] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables > > this message. > > [13201.078830] ksmd D ffff883f65eb7dc8 0 351 2 > > 0x00000000 > > [13201.085903] ffff883f65eb7dc8 ffff887f66e26400 ffff883f65d5e400 > > ffff883f65eb8000 > > [13201.093343] ffffffff81a65144 ffff883f65d5e400 00000000ffffffff > > ffffffff81a65148 > > [13201.100792] ffff883f65eb7de0 ffffffff816907e5 ffffffff81a65140 > > ffff883f65eb7df0 > > [13201.108242] Call Trace: > > [13201.110708] [<ffffffff816907e5>] schedule+0x35/0x80 > > [13201.115676] [<ffffffff81690ace>] schedule_preempt_disabled+0xe/0x10 > > [13201.122044] [<ffffffff81692524>] __mutex_lock_slowpath+0xb4/0x130 > > [13201.128237] [<ffffffff816925bf>] mutex_lock+0x1f/0x2f > > [13201.133395] [<ffffffff811debd2>] ksm_scan_thread+0x62/0x1f0 > > [13201.139068] [<ffffffff810c8ac0>] ? wait_woken+0x80/0x80 > > [13201.144391] [<ffffffff811deb70>] ? ksm_do_scan+0x1140/0x1140 > > [13201.150164] [<ffffffff810a4378>] kthread+0xd8/0xf0 > > [13201.155056] [<ffffffff810a42a0>] ? kthread_park+0x60/0x60 > > [13201.160551] [<ffffffff8169460f>] ret_from_fork+0x3f/0x70 > > [13201.165961] [<ffffffff810a42a0>] ? kthread_park+0x60/0x60 > > > > It seems this is not connected with the new code, but it would be nice to > > also make unmerge_and_remove_all_rmap_items() more scheduler friendly. > > Agreed. I run echo 2 many times here with big stable_node chains but > this one never happened here, it likely shows easier on the 1TiB. I thought the above was a problem with "scheduler friendliness" in turn missing cond_resched() but the above is not a softlockup. The above can't be solved by improving scheduler friendliness, we didn't prevent the schedule for 120sec, just the mutex_lock was stuck and a stuck was in D state for too long, which in the KSM case for servers would be just a false positive. KSM would immediately stop after it takes the mutex anyway so the above only informs that we didn't run try_to_freeze() fast enough. The only trouble there could be with suspend for non-server usage. To hide the above (and reach try_to_freeze() quick) we could just do trylock in ksm_scan_thread and mutex_lock_interruptible() in the other places, but that still leaves the uninterruptible wait_on_bit to solve. Improving scheduler friendliness would have been more important than avoiding the above. remove_node_from_stable_tree would also do a cond_resched() if the rmap_item list is not empty so it was unlikely it could generate a softlockup for 120sec even with an enormous chain. However just like the &migrate_nodes list walk and like the remove_stable_node_chain caller both do a cond_resched() after remove_stable_node(), it sounds better to do it inside remove_stable_node_chain too in case we run into a chain and we need to free the dups. Just the previous patch won't help with the above. -- 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>