d/freebsd-skq
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

Fix two bugs that could result in PMC sampling effectively stopping.

Browse Source 
In both cases, the the effect of the bug was that a very small positive
number was written to the counter. This means that a large number of
events needed to occur before the next sampling interrupt would trigger.
Even with very frequently occurring events like clock cycles wrapping all
the way around could take a long time. Both bugs occurred when updating
the saved reload count for an outgoing thread on a context switch.

First, the counter-independent code compares the current reload count
against the count set when the thread switched in and generates a delta
to apply to the saved count. If this delta causes the reload counter
to go negative, it would add a full reload interval to wrap it around to
a positive value. The fix is to add the full reload interval if the
resulting counter is zero.

Second, occasionally the raw counter value read during a context switch
has actually wrapped, but an interrupt has not yet triggered. In this
case the existing logic would return a very large reload count (e.g.
2^48 - 2 if the counter had overflowed by a count of 2). This was seen
both for fixed-function and programmable counters on an E5-2643.
Workaround this case by returning a reload count of zero.

PR:		198149
Differential Revision:	https://reviews.freebsd.org/D2557
Reviewed by:	emaste
MFC after:	1 week
Sponsored by:	Norse Corp, Inc.
This commit is contained in:
John Baldwin 2015-05-19 19:15:19 +00:00
parent 2b1df86c17
commit a1febbf667
2 changed files with 9 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
sys/dev/hwpmc/hwpmc_core.c
View File

@ -203,6 +203,10 @@ core_pcpu_fini(struct pmc_mdep *md, int cpu)
static pmc_value_t
iaf_perfctr_value_to_reload_count(pmc_value_t v)
{
/* If the PMC has overflowed, return a reload count of zero. */
if ((v & (1ULL << (core_iaf_width - 1))) == 0)
return (0);
v &= (1ULL << core_iaf_width) - 1;
return (1ULL << core_iaf_width) - v;
}
@ -1806,6 +1810,10 @@ static const int niap_events = sizeof(iap_events) / sizeof(iap_events[0]);
static pmc_value_t
iap_perfctr_value_to_reload_count(pmc_value_t v)
{
/* If the PMC has overflowed, return a reload count of zero. */
if ((v & (1ULL << (core_iap_width - 1))) == 0)
return (0);
v &= (1ULL << core_iap_width) - 1;
return (1ULL << core_iap_width) - v;
}

2
sys/dev/hwpmc/hwpmc_mod.c
View File

@ -1435,7 +1435,7 @@ pmc_process_csw_out(struct thread *td)
tmp += pm->pm_sc.pm_reloadcount;
mtx_pool_lock_spin(pmc_mtxpool, pm);
pp->pp_pmcs[ri].pp_pmcval -= tmp;
if ((int64_t) pp->pp_pmcs[ri].pp_pmcval < 0)
if ((int64_t) pp->pp_pmcs[ri].pp_pmcval <= 0)
pp->pp_pmcs[ri].pp_pmcval +=
pm->pm_sc.pm_reloadcount;
mtx_pool_unlock_spin(pmc_mtxpool, pm);