Re: [PATCH 2/4] mm: Send one IPI per CPU to TLB flush all entries after unmapping pages

Mel Gorman
2015-06-10T08:51:45Z

On Wed, Jun 10, 2015 at 10:21:07AM +0200, Ingo Molnar wrote:
>
> * Mel Gorman <mgorman@suse.de> wrote:
>
> > On Wed, Jun 10, 2015 at 09:47:04AM +0200, Ingo Molnar wrote:
> > >
> > > * Mel Gorman <mgorman@suse.de> wrote:
> > >
> > > > --- a/include/linux/sched.h
> > > > +++ b/include/linux/sched.h
> > > > @@ -1289,6 +1289,18 @@ enum perf_event_task_context {
> > > > perf_nr_task_contexts,
> > > > };
> > > >
> > > > +/* Track pages that require TLB flushes */
> > > > +struct tlbflush_unmap_batch {
> > > > + /*
> > > > + * Each bit set is a CPU that potentially has a TLB entry for one of
> > > > + * the PFNs being flushed. See set_tlb_ubc_flush_pending().
> > > > + */
> > > > + struct cpumask cpumask;
> > > > +
> > > > + /* True if any bit in cpumask is set */
> > > > + bool flush_required;
> > > > +};
> > > > +
> > > > struct task_struct {
> > > > volatile long state; /* -1 unrunnable, 0 runnable, >0 stopped */
> > > > void *stack;
> > > > @@ -1648,6 +1660,10 @@ struct task_struct {
> > > > unsigned long numa_pages_migrated;
> > > > #endif /* CONFIG_NUMA_BALANCING */
> > > >
> > > > +#ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
> > > > + struct tlbflush_unmap_batch *tlb_ubc;
> > > > +#endif
> > >
> > > Please embedd this constant size structure in task_struct directly so that the
> > > whole per task allocation overhead goes away:
> > >
> >
> > That puts a structure (72 bytes in the config I used) within the task struct
> > even when it's not required. On a lightly loaded system direct reclaim will not
> > be active and for some processes, it'll never be active. It's very wasteful.
>
> For certain values of 'very'.
>
> - 72 bytes suggests that you have NR_CPUS set to 512 or so? On a kernel sized to
> such large systems with 1000 active tasks we are talking about about +72K of
> RAM...
>

The NR_CPUS is based on the openSUSE 13.1 distro config so yes, it's large but I also
expect it to be a common configuration.

> - Furthermore, by embedding it it gets packed better with neighboring task_struct
> fields, while by allocating it dynamically it's a separate cache line wasted.
>

A separate cache line that is only used during direct reclaim when the
process is taking a large hit anyway

> - Plus by allocating it separately you spend two cachelines on it: each slab will
> be at least cacheline aligned, and 72 bytes will allocate 128 bytes. So when
> this gets triggered you've just wasted some more RAM.
>
> - I mean, if it had dynamic size, or was arguably huge. But this is just a
> cpumask and a boolean!
>

It gets larger with enterprise configs.

> - The cpumask will be dynamic if you increase the NR_CPUS count any more than
> that - in which case embedding the structure is the right choice again.
>

Enterprise configurations are larger. The most recent one I checked defined
NR_CPUS as 8192. If it's embedded in the structure, it means that we need
to call cpumask_clear on every fork even if it's never used. That adds
constant overhead to a fast path to avoid an allocation and a few cache
misses in a direct reclaim path. Are you certain you want that trade-off?

--
Mel Gorman
SUSE Labs