Messages in this thread | | | Date | Wed, 22 Mar 2006 10:05:56 -0800 | From | Paul Jackson <> | Subject | Re: [PATCH] Cpuset: alloc_pages_node overrides cpuset constraints |
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Andi wrote: > Faster would be if (gfp_mask & (__GFP_NOCPUSET|__GFP_HARDWALL)) { /* sort them out */ }
Yup - good point. However ...
Note that I am already off the fast path here, having peeled off the interrupt and node inside cpuset cases above. After these checks for __GFP_NOCPUSET or __GFP_HARDWALL, only the rare case of having to look outside a cpuset with no free memory for essential kernel memory remains.
Look at this entire routine:
int __cpuset_zone_allowed(struct zone *z, gfp_t gfp_mask) { int node; /* node that zone z is on */ const struct cpuset *cs; /* current cpuset ancestors */ int allowed; /* is allocation in zone z allowed? */
if (in_interrupt()) return 1; node = z->zone_pgdat->node_id; if (node_isset(node, current->mems_allowed)) return 1; if (gfp_mask & __GFP_NOCPUSET) return 1; if (gfp_mask & __GFP_HARDWALL) /* If hardwall request, stop here */ return 0;
if (current->flags & PF_EXITING) /* Let dying task have memory */ return 1;
/* Not hardwall and node outside mems_allowed: scan up cpusets */ mutex_lock(&callback_mutex);
task_lock(current); cs = nearest_exclusive_ancestor(current->cpuset); task_unlock(current);
allowed = node_isset(node, cs->mems_allowed); mutex_unlock(&callback_mutex); return allowed; }
Notice that if neither of the __GFP_NOCPUSET or __GFP_HARDWALL flag tests fire, then the code hits a mutex, spinlock and subroutine call.
Also notice that the __GFP_NOCPUSET case is the most important case of those at or below that check. Any alloc_pages_node, zmalloc_node or kmalloc_node call that requires a node outside the cpuset hits that case. Only tasks that have used up all the available memory in their cpuset commonly get past here, to the __GFP_HARDWALL case, which is not a case worth optimizing at the expense of more important code paths.
So ... actually ... I suspect that doing:
if (gfp_mask & __GFP_NOCPUSET) return 1; if (gfp_mask & __GFP_HARDWALL) return 0;
is faster than doing:
if (gfp_mask & (__GFP_NOCPUSET|__GFP_HARDWALL)) { if (gfp_mask & __GFP_NOCPUSET) return 1; if (gfp_mask & __GFP_HARDWALL) return 0; }
because the first of these two gets to the relatively more important case of __GFP_NOCPUSET faster.
Too bad the patch didn't show a little more context.
-- I won't rest till it's the best ... Programmer, Linux Scalability Paul Jackson <pj@sgi.com> 1.925.600.0401 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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