On Wed 03-03-21 21:18:32, Feng Tang wrote: > On Wed, Mar 03, 2021 at 01:32:11PM +0100, Michal Hocko wrote: > > On Wed 03-03-21 20:18:33, Feng Tang wrote: > > > On Wed, Mar 03, 2021 at 08:07:17PM +0800, Tang, Feng wrote: > > > > Hi Michal, > > > > > > > > On Wed, Mar 03, 2021 at 12:39:57PM +0100, Michal Hocko wrote: > > > > > On Wed 03-03-21 18:20:58, Feng Tang wrote: > > > > > > When doing broader test, we noticed allocation slowness in one test > > > > > > case that malloc memory with size which is slightly bigger than free > > > > > > memory of targeted nodes, but much less then the total free memory > > > > > > of system. > > > > > > > > > > > > The reason is the code enters the slowpath of __alloc_pages_nodemask(), > > > > > > which takes quite some time. As alloc_pages_policy() will give it a 2nd > > > > > > try with NULL nodemask, so there is no need to enter the slowpath for > > > > > > the first try. Add a new gfp bit to skip the slowpath, so that user cases > > > > > > like this can leverage. > > > > > > > > > > > > With it, the malloc in such case is much accelerated as it never enters > > > > > > the slowpath. > > > > > > > > > > > > Adding a new gfp_mask bit is generally not liked, and another idea is to > > > > > > add another nodemask to struct 'alloc_context', so it has 2: 'preferred-nmask' > > > > > > and 'fallback-nmask', and they will be tried in turn if not NULL, with > > > > > > it we can call __alloc_pages_nodemask() only once. > > > > > > > > > > Yes, it is very much disliked. Is there any reason why you cannot use > > > > > GFP_NOWAIT for that purpose? > > > > > > > > I did try that at the first place, but it didn't obviously change the slowness. > > > > I assumed the direct claim was still involved as GFP_NOWAIT only impact kswapd > > > > reclaim. > > > > I assume you haven't really created gfp mask correctly. What was the > > exact gfp mask you have used? > > The testcase is a malloc with multi-preferred-node policy, IIRC, the gfp > mask is HIGHUSER_MOVABLE originally, and code here ORs (__GFP_RETRY_MAYFAIL | __GFP_NOWARN). > > As GFP_WAIT == __GFP_KSWAPD_RECLAIM, in this test case, the bit is already set. Yes, you have to clear the gfp flag for the direct reclaim. I can see how that can be confusing though > > > One thing I tried which can fix the slowness is: > > > > > > + gfp_mask &= ~(__GFP_DIRECT_RECLAIM | __GFP_KSWAPD_RECLAIM); > > > > > > which explicitly clears the 2 kinds of reclaim. And I thought it's too > > > hacky and didn't mention it in the commit log. > > > > Clearing __GFP_DIRECT_RECLAIM would be the right way to achieve > > GFP_NOWAIT semantic. Why would you want to exclude kswapd as well? > > When I tried gfp_mask &= ~__GFP_DIRECT_RECLAIM, the slowness couldn't > be fixed. OK, I thought that you wanted to prevent the direct reclaim because that is the usual suspect for a slow down. If this is not not related to the direct reclaim then please try to find out what the acutal bottle neck is. Also how big of a slowdown are we talking about here? -- Michal Hocko SUSE Labs
