Re: alloc_pages_bulk()

[Chuck Lever <chuck.lever@oracle.com>]

> On Feb 15, 2021, at 7:06 AM, Mel Gorman <mgorman@techsingularity.net> wrote:
> 
> On Thu, Feb 11, 2021 at 04:20:31PM +0000, Chuck Lever wrote:
>>> On Feb 11, 2021, at 4:12 AM, Mel Gorman <mgorman@techsingularity.net> wrote:
>>> 
>>> <SNIP>
>>> 
>>> Parameters to __rmqueue_pcplist are garbage as the parameter order changed.
>>> I'm surprised it didn't blow up in a spectacular fashion. Again, this
>>> hasn't been near any testing and passing a list with high orders to
>>> free_pages_bulk() will corrupt lists too. Mostly it's a curiousity to see
>>> if there is justification for reworking the allocator to fundamentally
>>> deal in batches and then feed batches to pcp lists and the bulk allocator
>>> while leaving the normal GFP API as single page "batches". While that
>>> would be ideal, it's relatively high risk for regressions. There is still
>>> some scope for adding a basic bulk allocator before considering a major
>>> refactoring effort.
>>> 
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index f8353ea7b977..8f3fe7de2cf7 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -5892,7 +5892,7 @@ __alloc_pages_bulk_nodemask(gfp_t gfp_mask, unsigned int order,
>>> 	pcp_list = &pcp->lists[migratetype];
>>> 
>>> 	while (nr_pages) {
>>> -		page = __rmqueue_pcplist(zone, gfp_mask, migratetype,
>>> +		page = __rmqueue_pcplist(zone, migratetype, alloc_flags,
>>> 								pcp, pcp_list);
>>> 		if (!page)
>>> 			break;
>> 
>> The NFS server is considerably more stable now. Thank you!
>> 
> 
> Thanks for testing!
> 
>> I confirmed that my patch is requesting and getting multiple pages.
>> The new NFSD code and the API seem to be working as expected.
>> 
>> The results are stunning. Each svc_alloc_arg() call here allocates
>> 65 pages to satisfy a 256KB NFS READ request.
>> 
>> Before:
>> 
>>            nfsd-972   [000]   584.513817: funcgraph_entry:      + 35.385 us  |  svc_alloc_arg();
>>            nfsd-979   [002]   584.513870: funcgraph_entry:      + 29.051 us  |  svc_alloc_arg();
>>            nfsd-980   [001]   584.513951: funcgraph_entry:      + 29.178 us  |  svc_alloc_arg();
>>            nfsd-983   [000]   584.514014: funcgraph_entry:      + 29.211 us  |  svc_alloc_arg();
>>            nfsd-976   [002]   584.514059: funcgraph_entry:      + 29.315 us  |  svc_alloc_arg();
>>            nfsd-974   [001]   584.514127: funcgraph_entry:      + 29.237 us  |  svc_alloc_arg();
>> 
>> After:
>> 
>>            nfsd-977   [002]    87.049425: funcgraph_entry:        4.293 us   |  svc_alloc_arg();
>>            nfsd-981   [000]    87.049478: funcgraph_entry:        4.059 us   |  svc_alloc_arg();
>>            nfsd-988   [001]    87.049549: funcgraph_entry:        4.474 us   |  svc_alloc_arg();
>>            nfsd-983   [003]    87.049612: funcgraph_entry:        3.819 us   |  svc_alloc_arg();
>>            nfsd-976   [000]    87.049619: funcgraph_entry:        3.869 us   |  svc_alloc_arg();
>>            nfsd-980   [002]    87.049738: funcgraph_entry:        4.124 us   |  svc_alloc_arg();
>>            nfsd-975   [000]    87.049769: funcgraph_entry:        3.734 us   |  svc_alloc_arg();
>> 
> 
> Uhhhh, that is much better than I expected given how lame the
> implementation is.

My experience with function tracing is the entry and exit
timestamping adds significant overhead. I'd bet the actual
timing improvement is still good but much less.


> Sure -- it works, but it has more overhead than it
> should with the downside that reducing it requires fairly deep surgery. It
> may be enough to tidy this up to handle order-0 pages only to start with
> and see how far it gets. That's a fairly trivial modification.

I'd like to see an "order-0 only" implementation go in soon.
The improvement is palpable and there are several worthy
consumers on deck.


>> There appears to be little cost change for single-page allocations
>> using the bulk allocator (nr_pages=1):
>> 
>> Before:
>> 
>>            nfsd-985   [003]   572.324517: funcgraph_entry:        0.332 us   |  svc_alloc_arg();
>>            nfsd-986   [001]   572.324531: funcgraph_entry:        0.311 us   |  svc_alloc_arg();
>>            nfsd-985   [003]   572.324701: funcgraph_entry:        0.311 us   |  svc_alloc_arg();
>>            nfsd-986   [001]   572.324727: funcgraph_entry:        0.424 us   |  svc_alloc_arg();
>>            nfsd-985   [003]   572.324760: funcgraph_entry:        0.332 us   |  svc_alloc_arg();
>>            nfsd-986   [001]   572.324786: funcgraph_entry:        0.390 us   |  svc_alloc_arg();
>> 
>> After:
>> 
>>            nfsd-989   [002]    75.043226: funcgraph_entry:        0.322 us   |  svc_alloc_arg();
>>            nfsd-988   [001]    75.043436: funcgraph_entry:        0.368 us   |  svc_alloc_arg();
>>            nfsd-989   [002]    75.043464: funcgraph_entry:        0.424 us   |  svc_alloc_arg();
>>            nfsd-988   [001]    75.043490: funcgraph_entry:        0.317 us   |  svc_alloc_arg();
>>            nfsd-989   [002]    75.043517: funcgraph_entry:        0.425 us   |  svc_alloc_arg();
>>            nfsd-988   [001]    75.050025: funcgraph_entry:        0.407 us   |  svc_alloc_arg();
>> 
> 
> That is not too surprising given that there would be some additional
> overhead to manage a list of 1 page. I would hope that users of the bulk
> allocator are not routinely calling it with nr_pages == 1.

The NFSD implementation I did uses alloc_pages_bulk() to fill however
many pages are needed. Often that's just one page.

Sometimes it's zero pages. alloc_pages_bulk() does not behave very
well, so NFSD avoids calling it in that case.

I can post the patch for review. I cleaned it up recently but haven't
had a chance to test the clean-ups, so it might not work in its
current state.

--
Chuck Lever