Re: Chasing OOM Issues - good sysctl metrics to use?

From: Mark Millard <marklmi_at_yahoo.com>
Date: Wed, 11 May 2022 00:49:49 UTC
On 2022-May-10, at 11:49, Mark Millard <marklmi@yahoo.com> wrote:

> On 2022-May-10, at 08:47, Jan Mikkelsen <janm@transactionware.com> wrote:
> 
>> On 10 May 2022, at 10:01, Mark Millard <marklmi@yahoo.com> wrote:
>>> 
>>> On 2022-Apr-29, at 13:57, Mark Millard <marklmi@yahoo.com> wrote:
>>> 
>>>> On 2022-Apr-29, at 13:41, Pete Wright <pete@nomadlogic.org> wrote:
>>>>> 
>>>>>> . . .
>>>>> 
>>>>> d'oh - went out for lunch and workstation locked up.  i *knew* i shouldn't have said anything lol.
>>>> 
>>>> Any interesting console messages ( or dmesg -a or /var/log/messages )?
>>>> 
>>> 
>>> I've been doing some testing of a patch by tijl at FreeBSD.org
>>> and have reproduced both hang-ups (ZFS/ARC context) and kills
>>> (UFS/noARC and ZFS/ARC) for "was killed: failed to reclaim
>>> memory", both with and without the patch. This is with only a
>>> tiny fraction of the swap partition(s) enabled being put to
>>> use. So far, the testing was deliberately with
>>> vm.pageout_oom_seq=12 (the default value). My testing has been
>>> with main [so: 14].
>>> 
>>> But I also learned how to avoid the hang-ups that I got --but
>>> it costs making kills more likely/quicker, other things being
>>> equal.
>>> 
>>> I discovered that the hang-ups that I got were from all the
>>> processes that I interact with the system via ending up with
>>> the process's kernel threads swapped out and were not being
>>> swapped in. (including sshd, so no new ssh connections). In
>>> some contexts I only had escaping into the kernel debugger
>>> available, not even ^T would work. Other times ^T did work.
>>> 
>>> So, when I'm willing to risk kills in order to maintain
>>> the ability to interact normally, I now use in
>>> /etc/sysctl.conf :
>>> 
>>> vm.swap_enabled=0
>> 
>> I have been looking at an OOM related issue. Ignoring the actual leak, the problem leads to a process being killed because the system was out of memory. This is fine. After that, however, the system console was black with a single block cursor and the console keyboard was unresponsive. Caps lock and num lock didn’t toggle their lights when pressed.
>> 
>> Using an ssh session, the system looked fine. USB events for the keyboard being disconnected and reconnected appeared but the keyboard stayed unresponsive.
>> 
>> Setting vm.swap_enabled=0, as you did above, resolved this problem. After the process was killed a perfectly normal console returned.
>> 
>> The interesting thing is that this test system is configured with no swap space.
>> 
>> This is on 13.1-RC5.
>> 
>>> This disables swapping out of process kernel stacks. It
>>> is just with that option removedfor gaining free RAM, there
>>> fewer options tried before a kill is initiated. It is not a
>>> loader-time tunable but is writable, thus the
>>> /etc/sysctl.conf placement.
>> 
>> Is that really what it does? From a quick look at the code in vm/vm_swapout.c, it seems little more complex.
> 
> I was going by its description:
> 
> # sysctl -d vm.swap_enabled
> vm.swap_enabled: Enable entire process swapout
> 
> Based on the below, it appears that the description
> presumes vm.swap_idle_enabled==0 (the default). In
> my context vm.swap_idle_enabled==0 . Looks like I
> should also list:
> 
> vm.swap_idle_enabled=0
> 
> in my /etc/sysctl.conf with a reminder comment that the
> pair of =0's are required for avoiding the observed
> hang-ups.
> 
> 
> The  analysis goes like . . .
> 
> I see in the code that vm.swap_enabled !=0 causes
> VM_SWAP_NORMAL :
> 
> void
> vm_swapout_run(void)
> {
> 
>        if (vm_swap_enabled)
>                vm_req_vmdaemon(VM_SWAP_NORMAL);
> }
> 
> and that in turn leads to vm_daemon to:
> 
>                if (swapout_flags != 0) {
>                        /*
>                         * Drain the per-CPU page queue batches as a deadlock
>                         * avoidance measure.
>                         */
>                        if ((swapout_flags & VM_SWAP_NORMAL) != 0)
>                                vm_page_pqbatch_drain();
>                        swapout_procs(swapout_flags);
>                }
> 
> Note: vm.swap_idle_enabled==0 && vm.swap_enabled==0 ends
> up with swapout_flags==0. vm.swap_idle. . . defaults seem
> to be (in my context):
> 
> # sysctl -a | grep swap_idle
> vm.swap_idle_threshold2: 10
> vm.swap_idle_threshold1: 2
> vm.swap_idle_enabled: 0
> 
> For reference:
> 
> /*
> * Idle process swapout -- run once per second when pagedaemons are
> * reclaiming pages.
> */
> void
> vm_swapout_run_idle(void)
> {
>        static long lsec;
> 
>        if (!vm_swap_idle_enabled || time_second == lsec)
>                return;
>        vm_req_vmdaemon(VM_SWAP_IDLE);
>        lsec = time_second;
> }
> 
> [So vm.swap_idle_enabled==0 avoids VM_SWAP_IDLE status.]
> 
> static void
> vm_req_vmdaemon(int req)
> {
>        static int lastrun = 0;
> 
>        mtx_lock(&vm_daemon_mtx);
>        vm_pageout_req_swapout |= req;
>        if ((ticks > (lastrun + hz)) || (ticks < lastrun)) {
>                wakeup(&vm_daemon_needed);
>                lastrun = ticks;
>        }
>        mtx_unlock(&vm_daemon_mtx);
> }
> 
> [So VM_SWAP_IDLE and VM_SWAP_NORMAL are independent bits
> in vm_pageout_req_swapout.]
> 
> vm_deamon does:
> 
>                mtx_lock(&vm_daemon_mtx);
>                msleep(&vm_daemon_needed, &vm_daemon_mtx, PPAUSE, "psleep",
>                    vm_daemon_timeout);
>                swapout_flags = vm_pageout_req_swapout;
>                vm_pageout_req_swapout = 0;
>                mtx_unlock(&vm_daemon_mtx);
> 
> So vm_pageout_req_swapout is regenerated after thata
> each time.
> 
> I'll not show the code for vm.swap_idle_enabled!=0 .
> 

Well, with continued experiments I got an example of
a hangup for which looking via the db> prompt did not
show any swapping out of process kernel stacks
( vm.swap_enabled=0 was the context, so expected ).
The environment was ZFS (so with ARC).

But this was testing with vm.pageout_oom_seq=120 instead
of the default vm.pageout_oom_seq=12 . It may be that
let sit long enough things would have unhung (external
perspective).

It is part of what I'm experimenting with so we will see.



===
Mark Millard
marklmi at yahoo.com