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credit2: fix credit reset happening too few times
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There is a bug in commit 5e4b419 ("xen: credit2: only reset
credit on reset condition"). In fact, the aim of that commit was to
make sure that we do not perform too many credit reset operations
(which are not super cheap, and in an hot-path). But the check used
to determine whether a reset is necessary was the wrong one.

In fact, knowing just that some vCPUs have been skipped, while
traversing the runqueue (in runq_candidate()), is not enough. We
need to check explicitly whether the first vCPU in the runqueue
has a negative amount of credit.

Since a trace record is changed, this patch updates xentrace format file
and xenalyze as well

This should be backported.

Signed-off-by: Dario Faggioli <[email protected]>
Acked-by: George Dunlap <[email protected]>
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@dfaggioli @jbeulich
dfaggioli authored and jbeulich committed Apr 3, 2020
1 parent 36f3662 commit dae7b62
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Showing 3 changed files with 17 additions and 23 deletions.
2 changes: 1 addition & 1 deletion tools/xentrace/formats
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Original file line number Diff line number Diff line change
Expand Up @@ -67,7 +67,7 @@
0x00022210 CPU%(cpu)d %(tsc)d (+%(reltsc)8d) csched2:load_check [ lrq_id[16]:orq_id[16] = 0x%(1)08x, delta = %(2)d ]
0x00022211 CPU%(cpu)d %(tsc)d (+%(reltsc)8d) csched2:load_balance [ l_bavgload = 0x%(2)08x%(1)08x, o_bavgload = 0x%(4)08x%(3)08x, lrq_id[16]:orq_id[16] = 0x%(5)08x ]
0x00022212 CPU%(cpu)d %(tsc)d (+%(reltsc)8d) csched2:pick_cpu [ b_avgload = 0x%(2)08x%(1)08x, dom:vcpu = 0x%(3)08x, rq_id[16]:new_cpu[16] = %(4)d ]
0x00022213 CPU%(cpu)d %(tsc)d (+%(reltsc)8d) csched2:runq_candidate [ dom:vcpu = 0x%(1)08x, credit = %(4)d, skipped_vcpus = %(3)d, tickled_cpu = %(2)d ]
0x00022213 CPU%(cpu)d %(tsc)d (+%(reltsc)8d) csched2:runq_candidate [ dom:vcpu = 0x%(1)08x, credit = %(3)d, tickled_cpu = %(2)d ]
0x00022214 CPU%(cpu)d %(tsc)d (+%(reltsc)8d) csched2:schedule [ rq:cpu = 0x%(1)08x, tasklet[8]:idle[8]:smt_idle[8]:tickled[8] = %(2)08x ]
0x00022215 CPU%(cpu)d %(tsc)d (+%(reltsc)8d) csched2:ratelimit [ dom:vcpu = 0x%(1)08x, runtime = %(2)d ]
0x00022216 CPU%(cpu)d %(tsc)d (+%(reltsc)8d) csched2:runq_cand_chk [ dom:vcpu = 0x%(1)08x ]
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8 changes: 3 additions & 5 deletions tools/xentrace/xenalyze.c
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Original file line number Diff line number Diff line change
Expand Up @@ -7855,14 +7855,12 @@ void sched_process(struct pcpu_info *p)
if (opt.dump_all) {
struct {
unsigned vcpuid:16, domid:16;
unsigned tickled_cpu, skipped;
unsigned tickled_cpu;
int credit;
} *r = (typeof(r))ri->d;

printf(" %s csched2:runq_candidate d%uv%u, credit = %d, "
"%u vcpus skipped, ",
ri->dump_header, r->domid, r->vcpuid,
r->credit, r->skipped);
printf(" %s csched2:runq_candidate d%uv%u, credit = %d, ",
ri->dump_header, r->domid, r->vcpuid, r->credit);
if (r->tickled_cpu == (unsigned)-1)
printf("no cpu was tickled\n");
else
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30 changes: 13 additions & 17 deletions xen/common/sched/credit2.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3224,17 +3224,14 @@ csched2_runtime(const struct scheduler *ops, int cpu,
static struct csched2_unit *
runq_candidate(struct csched2_runqueue_data *rqd,
struct csched2_unit *scurr,
int cpu, s_time_t now,
unsigned int *skipped)
int cpu, s_time_t now)
{
struct list_head *iter, *temp;
const struct sched_resource *sr = get_sched_res(cpu);
struct csched2_unit *snext = NULL;
struct csched2_private *prv = csched2_priv(sr->scheduler);
bool yield = false, soft_aff_preempt = false;

*skipped = 0;

if ( unlikely(is_idle_unit(scurr->unit)) )
{
snext = scurr;
Expand Down Expand Up @@ -3328,12 +3325,9 @@ runq_candidate(struct csched2_runqueue_data *rqd,
(unsigned char *)&d);
}

/* Only consider units that are allowed to run on this processor. */
/* Only consider vcpus that are allowed to run on this processor. */
if ( !cpumask_test_cpu(cpu, svc->unit->cpu_hard_affinity) )
{
(*skipped)++;
continue;
}

/*
* If an unit is meant to be picked up by another processor, and such
Expand All @@ -3342,7 +3336,6 @@ runq_candidate(struct csched2_runqueue_data *rqd,
if ( svc->tickled_cpu != -1 && svc->tickled_cpu != cpu &&
cpumask_test_cpu(svc->tickled_cpu, &rqd->tickled) )
{
(*skipped)++;
SCHED_STAT_CRANK(deferred_to_tickled_cpu);
continue;
}
Expand All @@ -3354,7 +3347,6 @@ runq_candidate(struct csched2_runqueue_data *rqd,
if ( sched_unit_master(svc->unit) != cpu
&& snext->credit + CSCHED2_MIGRATE_RESIST > svc->credit )
{
(*skipped)++;
SCHED_STAT_CRANK(migrate_resisted);
continue;
}
Expand All @@ -3378,14 +3370,13 @@ runq_candidate(struct csched2_runqueue_data *rqd,
{
struct {
unsigned unit:16, dom:16;
unsigned tickled_cpu, skipped;
unsigned tickled_cpu;
int credit;
} d;
d.dom = snext->unit->domain->domain_id;
d.unit = snext->unit->unit_id;
d.credit = snext->credit;
d.tickled_cpu = snext->tickled_cpu;
d.skipped = *skipped;
__trace_var(TRC_CSCHED2_RUNQ_CANDIDATE, 1,
sizeof(d),
(unsigned char *)&d);
Expand Down Expand Up @@ -3417,7 +3408,6 @@ static void csched2_schedule(
struct csched2_runqueue_data *rqd;
struct csched2_unit * const scurr = csched2_unit(currunit);
struct csched2_unit *snext = NULL;
unsigned int skipped_units = 0;
bool tickled;
bool migrated = false;

Expand Down Expand Up @@ -3495,7 +3485,7 @@ static void csched2_schedule(
snext = csched2_unit(sched_idle_unit(sched_cpu));
}
else
snext = runq_candidate(rqd, scurr, sched_cpu, now, &skipped_units);
snext = runq_candidate(rqd, scurr, sched_cpu, now);

/* If switching from a non-idle runnable unit, put it
* back on the runqueue. */
Expand All @@ -3507,6 +3497,8 @@ static void csched2_schedule(
/* Accounting for non-idle tasks */
if ( !is_idle_unit(snext->unit) )
{
int top_credit;

/* If switching, remove this from the runqueue and mark it scheduled */
if ( snext != scurr )
{
Expand Down Expand Up @@ -3534,11 +3526,15 @@ static void csched2_schedule(
* 2) no other unit with higher credits wants to run.
*
* Here, where we want to check for reset, we need to make sure the
* proper unit is being used. In fact, runqueue_candidate() may have
* not returned the first unit in the runqueue, for various reasons
* proper unit is being used. In fact, runq_candidate() may have not
* returned the first unit in the runqueue, for various reasons
* (e.g., affinity). Only trigger a reset when it does.
*/
if ( skipped_units == 0 && snext->credit <= CSCHED2_CREDIT_RESET )
if ( list_empty(&rqd->runq) )
top_credit = snext->credit;
else
top_credit = max(snext->credit, runq_elem(rqd->runq.next)->credit);
if ( top_credit <= CSCHED2_CREDIT_RESET )
{
reset_credit(sched_cpu, now, snext);
balance_load(ops, sched_cpu, now);
Expand Down

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