Messages in this thread | | | From | Vitaly Kuznetsov <> | Subject | Re: [lkp-robot] [x86/mm] 9e52fc2b50: will-it-scale.per_thread_ops -16% regression | Date | Fri, 29 Sep 2017 16:02:27 +0200 |
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Peter Zijlstra <peterz@infradead.org> writes:
> 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. >>
(purely teoretical thought) what I meant to say is in tlb_remove_table() we may try to get new batch from some pre-allocated (on boot) pool and revert to __get_free_page() when it's empty. This may make sense combined with the next idea, allocating more than 1 page.
>> > 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.
Of course, but the question is: what's cheaper -- try to alloc e.g. 8 pages or do 8 smp_call_function() calls?
But adding such complexity to the code would require a good justification, of course.
-- Vitaly
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