Re: [PATCH v1 2/2] mm: add priority threshold to __purge_vmap_area_lazy()

[Joel Fernandes]

On Tue, Jan 29, 2019 at 06:39:36PM +0100, Uladzislau Rezki wrote:
> On Mon, Jan 28, 2019 at 05:45:28PM -0500, Joel Fernandes wrote:
> > On Thu, Jan 24, 2019 at 12:56:48PM +0100, Uladzislau Rezki (Sony) wrote:
> > > commit 763b218ddfaf ("mm: add preempt points into
> > > __purge_vmap_area_lazy()")
> > > 
> > > introduced some preempt points, one of those is making an
> > > allocation more prioritized over lazy free of vmap areas.
> > > 
> > > Prioritizing an allocation over freeing does not work well
> > > all the time, i.e. it should be rather a compromise.
> > > 
> > > 1) Number of lazy pages directly influence on busy list length
> > > thus on operations like: allocation, lookup, unmap, remove, etc.
> > > 
> > > 2) Under heavy stress of vmalloc subsystem i run into a situation
> > > when memory usage gets increased hitting out_of_memory -> panic
> > > state due to completely blocking of logic that frees vmap areas
> > > in the __purge_vmap_area_lazy() function.
> > > 
> > > Establish a threshold passing which the freeing is prioritized
> > > back over allocation creating a balance between each other.
> > 
> > I'm a bit concerned that this will introduce the latency back if vmap_lazy_nr
> > is greater than half of lazy_max_pages(). Which IIUC will be more likely if
> > the number of CPUs is large.
> > 
> The threshold that we establish is two times more than lazy_max_pages(),
> i.e. in case of 4 system CPUs lazy_max_pages() is 24576, therefore the
> threshold is 49152, if PAGE_SIZE is 4096.
> 
> It means that we allow rescheduling if vmap_lazy_nr < 49152. If vmap_lazy_nr 
> is higher then we forbid rescheduling and free areas until it becomes lower
> again to stabilize the system. By doing that, we will not allow vmap_lazy_nr
> to be enormously increased.

Sorry for late reply.

This sounds reasonable. Such an extreme situation of vmap_lazy_nr being twice
the lazy_max_pages() is probably only possible using a stress test anyway
since (hopefully) the try_purge_vmap_area_lazy() call is happening often
enough to keep the vmap_lazy_nr low.

Have you experimented with what is the highest threshold that prevents the
issues you're seeing? Have you tried 3x or 4x the vmap_lazy_nr?

I also wonder what is the cost these days of the global TLB flush on the most
common Linux architectures and if the whole purge vmap_area lazy stuff is
starting to get a bit dated, and if we can do the purging inline as areas are
freed. There is a cost to having this mechanism too as you said, which is as
the list size grows, all other operations also take time.

thanks,

 - Joel


> > In fact, when vmap_lazy_nr is high, that's when the latency will be the worst
> > so one could say that that's when you *should* reschedule since the frees are
> > taking too long and hurting real-time tasks.
> > 
> > Could this be better solved by tweaking lazy_max_pages() such that purging is
> > more aggressive?
> > 
> > Another approach could be to detect the scenario you brought up (allocations
> > happening faster than free), somehow, and avoid a reschedule?
> > 
> This is what i am trying to achieve by this change. 
> 
> Thank you for your comments.
> 
> --
> Vlad Rezki
> > > 
> > > Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
> > > ---
> > >  mm/vmalloc.c | 18 ++++++++++++------
> > >  1 file changed, 12 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > index fb4fb5fcee74..abe83f885069 100644
> > > --- a/mm/vmalloc.c
> > > +++ b/mm/vmalloc.c
> > > @@ -661,23 +661,27 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
> > >  	struct llist_node *valist;
> > >  	struct vmap_area *va;
> > >  	struct vmap_area *n_va;
> > > -	bool do_free = false;
> > > +	int resched_threshold;
> > >  
> > >  	lockdep_assert_held(&vmap_purge_lock);
> > >  
> > >  	valist = llist_del_all(&vmap_purge_list);
> > > +	if (unlikely(valist == NULL))
> > > +		return false;
> > > +
> > > +	/*
> > > +	 * TODO: to calculate a flush range without looping.
> > > +	 * The list can be up to lazy_max_pages() elements.
> > > +	 */
> > >  	llist_for_each_entry(va, valist, purge_list) {
> > >  		if (va->va_start < start)
> > >  			start = va->va_start;
> > >  		if (va->va_end > end)
> > >  			end = va->va_end;
> > > -		do_free = true;
> > >  	}
> > >  
> > > -	if (!do_free)
> > > -		return false;
> > > -
> > >  	flush_tlb_kernel_range(start, end);
> > > +	resched_threshold = (int) lazy_max_pages() << 1;
> > >  
> > >  	spin_lock(&vmap_area_lock);
> > >  	llist_for_each_entry_safe(va, n_va, valist, purge_list) {
> > > @@ -685,7 +689,9 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
> > >  
> > >  		__free_vmap_area(va);
> > >  		atomic_sub(nr, &vmap_lazy_nr);
> > > -		cond_resched_lock(&vmap_area_lock);
> > > +
> > > +		if (atomic_read(&vmap_lazy_nr) < resched_threshold)
> > > +			cond_resched_lock(&vmap_area_lock);
> > >  	}
> > >  	spin_unlock(&vmap_area_lock);
> > >  	return true;
> > > -- 
> > > 2.11.0
> > >