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Change vm_pageout_scan() to return a value indicating whether the free page

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target was met.

Previously, vm_pageout_worker() itself checked the length of the free page
queues to determine whether vm_pageout_scan(pass >= 1)'s inactive queue scan
freed enough pages to meet the free page target.  Specifically,
vm_pageout_worker() used vm_paging_needed().  The trouble with
vm_paging_needed() is that it compares the length of the free page queues to
the wakeup threshold for the page daemon, which is much lower than the free
page target.  Consequently, vm_pageout_worker() could conclude that the
inactive queue scan succeeded in meeting its free page target when in fact
it did not; and rather than immediately triggering an all-out laundering
pass over the inactive queue, vm_pageout_worker() would go back to sleep
waiting for the free page count to fall below the page daemon wakeup
threshold again, at which point it will perform another limited (pass == 1)
scan over the inactive queue.

Changing vm_pageout_worker() to use vm_page_count_target() instead of
vm_paging_needed() won't work because any page allocations that happen
concurrently with the inactive queue scan will result in the free page count
being below the target at the end of a successful scan.  Instead, having
vm_pageout_scan() return a value indicating success or failure is the most
straightforward fix.

Reviewed by:	kib, markj
MFC after:	3 weeks
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D8111
This commit is contained in:
Alan Cox 2016-10-05 16:15:26 +00:00
parent c5bf621b7a
commit e57dd910e6
1 changed files with 24 additions and 16 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

40
sys/vm/vm_pageout.c
View File

@ -121,7 +121,7 @@ static void vm_pageout(void);
static void vm_pageout_init(void);
static int vm_pageout_clean(vm_page_t m);
static int vm_pageout_cluster(vm_page_t m);
static void vm_pageout_scan(struct vm_domain *vmd, int pass);
static bool vm_pageout_scan(struct vm_domain *vmd, int pass);
static void vm_pageout_mightbe_oom(struct vm_domain *vmd, int page_shortage,
int starting_page_shortage);
@ -845,17 +845,20 @@ vm_pageout_clean(vm_page_t m)
* pass 0 - Update active LRU/deactivate pages
* pass 1 - Free inactive pages
* pass 2 - Launder dirty pages
*
* Returns true if pass was zero or enough pages were freed by the inactive
* queue scan to meet the target.
*/
static void
static bool
vm_pageout_scan(struct vm_domain *vmd, int pass)
{
vm_page_t m, next;
struct vm_pagequeue *pq;
vm_object_t object;
long min_scan;
int act_delta, addl_page_shortage, deficit, error, maxlaunder, maxscan;
int page_shortage, scan_tick, scanned, starting_page_shortage;
int vnodes_skipped;
int act_delta, addl_page_shortage, deficit, error, inactq_shortage;
int maxlaunder, maxscan, page_shortage, scan_tick, scanned;
int starting_page_shortage, vnodes_skipped;
boolean_t pageout_ok, queue_locked;
/*
@ -886,7 +889,9 @@ vm_pageout_scan(struct vm_domain *vmd, int pass)
addl_page_shortage = 0;
/*
* Calculate the number of pages that we want to free.
* Calculate the number of pages that we want to free. This number
* can be negative if many pages are freed between the wakeup call to
* the page daemon and this calculation.
*/
if (pass > 0) {
deficit = atomic_readandclear_int(&vm_pageout_deficit);
@ -956,7 +961,7 @@ vm_pageout_scan(struct vm_domain *vmd, int pass)
* Held pages are essentially stuck in the
* queue. So, they ought to be discounted
* from the inactive count. See the
* calculation of the page_shortage for the
* calculation of inactq_shortage before the
* loop over the active queue below.
*/
addl_page_shortage++;
@ -1164,7 +1169,7 @@ vm_pageout_scan(struct vm_domain *vmd, int pass)
* Compute the number of pages we want to try to move from the
* active queue to the inactive queue.
*/
page_shortage = vm_cnt.v_inactive_target - vm_cnt.v_inactive_count +
inactq_shortage = vm_cnt.v_inactive_target - vm_cnt.v_inactive_count +
vm_paging_target() + deficit + addl_page_shortage;
pq = &vmd->vmd_pagequeues[PQ_ACTIVE];
@ -1182,7 +1187,7 @@ vm_pageout_scan(struct vm_domain *vmd, int pass)
min_scan /= hz * vm_pageout_update_period;
} else
min_scan = 0;
if (min_scan > 0 || (page_shortage > 0 && maxscan > 0))
if (min_scan > 0 || (inactq_shortage > 0 && maxscan > 0))
vmd->vmd_last_active_scan = scan_tick;
/*
@ -1191,7 +1196,7 @@ vm_pageout_scan(struct vm_domain *vmd, int pass)
* candidates. Held pages may be deactivated.
*/
for (m = TAILQ_FIRST(&pq->pq_pl), scanned = 0; m != NULL && (scanned <
min_scan || (page_shortage > 0 && scanned < maxscan)); m = next,
min_scan || (inactq_shortage > 0 && scanned < maxscan)); m = next,
scanned++) {
KASSERT(m->queue == PQ_ACTIVE,
("vm_pageout_scan: page %p isn't active", m));
@ -1256,7 +1261,7 @@ vm_pageout_scan(struct vm_domain *vmd, int pass)
/* Dequeue to avoid later lock recursion. */
vm_page_dequeue_locked(m);
vm_page_deactivate(m);
page_shortage--;
inactq_shortage--;
} else
vm_page_requeue_locked(m);
vm_page_unlock(m);
@ -1275,6 +1280,7 @@ vm_pageout_scan(struct vm_domain *vmd, int pass)
}
}
#endif
return (page_shortage <= 0);
}
static int vm_pageout_oom_vote;
@ -1504,9 +1510,11 @@ vm_pageout_worker(void *arg)
{
struct vm_domain *domain;
int domidx;
bool target_met;
domidx = (uintptr_t)arg;
domain = &vm_dom[domidx];
target_met = true;
/*
* XXXKIB It could be useful to bind pageout daemon threads to
@ -1545,11 +1553,11 @@ vm_pageout_worker(void *arg)
}
/*
* Do not clear vm_pageout_wanted until we reach our target.
* Otherwise, we may be awakened over and over again, wasting
* CPU time.
* Do not clear vm_pageout_wanted until we reach our free page
* target. Otherwise, we may be awakened over and over again,
* wasting CPU time.
*/
if (vm_pageout_wanted && !vm_paging_needed())
if (vm_pageout_wanted && target_met)
vm_pageout_wanted = false;
/*
@ -1584,7 +1592,7 @@ vm_pageout_worker(void *arg)
domain->vmd_pass = 0;
}
vm_pageout_scan(domain, domain->vmd_pass);
target_met = vm_pageout_scan(domain, domain->vmd_pass);
}
}