Re: [PATCH 2/2] kvfree_rcu: Use same set of flags as for single-argument

[Uladzislau Rezki <urezki@gmail.com>]

On Thu, Feb 04, 2021 at 02:04:27PM -0800, Paul E. McKenney wrote:
> On Fri, Jan 29, 2021 at 09:05:05PM +0100, Uladzislau Rezki (Sony) wrote:
> > Running an rcuscale stress-suite can lead to "Out of memory"
> > of a system. This can happen under high memory pressure with
> > a small amount of physical memory.
> > 
> > For example a KVM test configuration with 64 CPUs and 512 megabytes
> > can lead to of memory after running rcuscale with below parameters:
> > 
> > ../kvm.sh --torture rcuscale --allcpus --duration 10 --kconfig CONFIG_NR_CPUS=64 \
> > --bootargs "rcuscale.kfree_rcu_test=1 rcuscale.kfree_nthreads=16 rcuscale.holdoff=20 \
> >   rcuscale.kfree_loops=10000 torture.disable_onoff_at_boot" --trust-make
> > 
> > <snip>
> > [   12.054448] kworker/1:1H invoked oom-killer: gfp_mask=0x2cc0(GFP_KERNEL|__GFP_NOWARN), order=0, oom_score_adj=0
> > [   12.055303] CPU: 1 PID: 377 Comm: kworker/1:1H Not tainted 5.11.0-rc3+ #510
> > [   12.055416] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.12.0-1 04/01/2014
> > [   12.056485] Workqueue: events_highpri fill_page_cache_func
> > [   12.056485] Call Trace:
> > [   12.056485]  dump_stack+0x57/0x6a
> > [   12.056485]  dump_header+0x4c/0x30a
> > [   12.056485]  ? del_timer_sync+0x20/0x30
> > [   12.056485]  out_of_memory.cold.47+0xa/0x7e
> > [   12.056485]  __alloc_pages_slowpath.constprop.123+0x82f/0xc00
> > [   12.056485]  __alloc_pages_nodemask+0x289/0x2c0
> > [   12.056485]  __get_free_pages+0x8/0x30
> > [   12.056485]  fill_page_cache_func+0x39/0xb0
> > [   12.056485]  process_one_work+0x1ed/0x3b0
> > [   12.056485]  ? process_one_work+0x3b0/0x3b0
> > [   12.060485]  worker_thread+0x28/0x3c0
> > [   12.060485]  ? process_one_work+0x3b0/0x3b0
> > [   12.060485]  kthread+0x138/0x160
> > [   12.060485]  ? kthread_park+0x80/0x80
> > [   12.060485]  ret_from_fork+0x22/0x30
> > [   12.062156] Mem-Info:
> > [   12.062350] active_anon:0 inactive_anon:0 isolated_anon:0
> > [   12.062350]  active_file:0 inactive_file:0 isolated_file:0
> > [   12.062350]  unevictable:0 dirty:0 writeback:0
> > [   12.062350]  slab_reclaimable:2797 slab_unreclaimable:80920
> > [   12.062350]  mapped:1 shmem:2 pagetables:8 bounce:0
> > [   12.062350]  free:10488 free_pcp:1227 free_cma:0
> > ...
> > [   12.101610] Out of memory and no killable processes...
> > [   12.102042] Kernel panic - not syncing: System is deadlocked on memory
> > [   12.102583] CPU: 1 PID: 377 Comm: kworker/1:1H Not tainted 5.11.0-rc3+ #510
> > [   12.102600] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.12.0-1 04/01/2014
> > <snip>
> > 
> > Having a fallback mechanism we should not go with "GFP_KERNEL | __GFP_NOWARN"
> > that implies a "hard" page request involving OOM killer. Replace such set with
> > the same as the one used for a single argument.
> > 
> > Thus it will follow same rules:
> >     a) minimize a fallback hitting;
> >     b) avoid of OOM invoking;
> >     c) do a light-wait page request;
> >     d) avoid of dipping into the emergency reserves.
> > 
> > With this change an rcuscale and the parameters which are in question
> > never runs into "Kernel panic".
> > 
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> 
> I did have some misgivings about this one, but after a closer look at
> the GFP flags you suggest along with offlist discussions it looks like
> what needs to happen.  So thank you for persisting!  ;-)
> 
> I did the usual wordsmithing as shown below, so please check to make
> sure that I did not mess anything up.
> 
Looks good to me, i mean a rewording of the commit message :)

--
Vlad Rezki