Re: [lkp-robot] [x86/mm] 9e52fc2b50: will-it-scale.per_thread_ops -16% regression

[Peter Zijlstra]

On Fri, Sep 29, 2017 at 03:13:29PM +0200, Peter Zijlstra wrote:
> On Fri, Sep 29, 2017 at 02:24:03PM +0200, Vitaly Kuznetsov wrote:
> > 1) In case the system is under extreme memory pressure and
> > __get_free_page() is failing in tlb_remove_table() we'll be doing
> > smp_call_function() for _each_ call (avoiding batching). We may want to
> > have a pre-allocated pool.
> 
> MMU_GATHER_BUNDLE should avoid it being for _every_ call.

My bad, that's only for pages, not tables :/

> Also, note that tlb_gather is preemptible, so pre-alloc is 'difficult'
> and you will run out, esp. when memory is right.
> 
> > 2) The default MAX_TABLE_BATCH is static (it is equal to the number of
> > pointer we can fit into one page - sizeof(struct mmu_table_batch) ==
> > 509), we may want to adjust it for very big systems.
> 
> That would then put more stress on the memory allocator because you're
> then asking for higher order pages.