Fix hwpmc "stalled" behavior
Currently, there is a single pm_stalled flag that tracks whether a performance monitor was "stalled" due to insufficent ring buffer space for samples. However, because the same performance monitor can run on multiple processes or threads at the same time, a single pm_stalled flag that impacts them all seems insufficient. In particular, you can hit corner cases where the code fails to stop performance monitors during a context switch out, because it thinks the performance monitor is already stopped. However, in reality, it may be that only the monitor running on a different CPU was stalled. This patch attempts to fix that behavior by tracking on a per-CPU basis whether a PM desires to run and whether it is "stalled". This lets the code make better decisions about when to stop PMs and when to try to restart them. Ideally, we should avoid the case where the code fails to stop a PM during a context switch out. Sponsored by: Juniper Networks Reviewed by: jhb Approved by: gnn (mentor) Differential Revision: https://reviews.freebsd.org/D4124
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@ -1298,6 +1298,15 @@ pmc_process_csw_in(struct thread *td)
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PMCDBG3(CSW,SWI,1,"cpu=%d ri=%d new=%jd", cpu, ri, newvalue);
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pcd->pcd_write_pmc(cpu, adjri, newvalue);
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/* If a sampling mode PMC, reset stalled state. */
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if (PMC_TO_MODE(pm) == PMC_MODE_TS)
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CPU_CLR_ATOMIC(cpu, &pm->pm_stalled);
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/* Indicate that we desire this to run. */
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CPU_SET_ATOMIC(cpu, &pm->pm_cpustate);
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/* Start the PMC. */
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pcd->pcd_start_pmc(cpu, adjri);
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}
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@ -1392,8 +1401,14 @@ pmc_process_csw_out(struct thread *td)
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("[pmc,%d] ri mismatch pmc(%d) ri(%d)",
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__LINE__, PMC_TO_ROWINDEX(pm), ri));
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/* Stop hardware if not already stopped */
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if (pm->pm_stalled == 0)
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/*
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* Change desired state, and then stop if not stalled.
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* This two-step dance should avoid race conditions where
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* an interrupt re-enables the PMC after this code has
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* already checked the pm_stalled flag.
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*/
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CPU_CLR_ATOMIC(cpu, &pm->pm_cpustate);
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if (!CPU_ISSET(cpu, &pm->pm_stalled))
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pcd->pcd_stop_pmc(cpu, adjri);
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/* reduce this PMC's runcount */
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@ -2258,8 +2273,9 @@ pmc_release_pmc_descriptor(struct pmc *pm)
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pmc_select_cpu(cpu);
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/* switch off non-stalled CPUs */
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CPU_CLR_ATOMIC(cpu, &pm->pm_cpustate);
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if (pm->pm_state == PMC_STATE_RUNNING &&
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pm->pm_stalled == 0) {
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!CPU_ISSET(cpu, &pm->pm_stalled)) {
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phw = pmc_pcpu[cpu]->pc_hwpmcs[ri];
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@ -2695,8 +2711,15 @@ pmc_start(struct pmc *pm)
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if ((error = pcd->pcd_write_pmc(cpu, adjri,
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PMC_IS_SAMPLING_MODE(mode) ?
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pm->pm_sc.pm_reloadcount :
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pm->pm_sc.pm_initial)) == 0)
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pm->pm_sc.pm_initial)) == 0) {
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/* If a sampling mode PMC, reset stalled state. */
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if (PMC_IS_SAMPLING_MODE(mode))
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CPU_CLR_ATOMIC(cpu, &pm->pm_stalled);
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/* Indicate that we desire this to run. Start it. */
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CPU_SET_ATOMIC(cpu, &pm->pm_cpustate);
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error = pcd->pcd_start_pmc(cpu, adjri);
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}
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critical_exit();
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pmc_restore_cpu_binding(&pb);
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@ -2758,6 +2781,7 @@ pmc_stop(struct pmc *pm)
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ri = PMC_TO_ROWINDEX(pm);
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pcd = pmc_ri_to_classdep(md, ri, &adjri);
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CPU_CLR_ATOMIC(cpu, &pm->pm_cpustate);
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critical_enter();
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if ((error = pcd->pcd_stop_pmc(cpu, adjri)) == 0)
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error = pcd->pcd_read_pmc(cpu, adjri, &pm->pm_sc.pm_initial);
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@ -4066,7 +4090,7 @@ pmc_process_interrupt(int cpu, int ring, struct pmc *pm, struct trapframe *tf,
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ps = psb->ps_write;
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if (ps->ps_nsamples) { /* in use, reader hasn't caught up */
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pm->pm_stalled = 1;
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CPU_SET_ATOMIC(cpu, &pm->pm_stalled);
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atomic_add_int(&pmc_stats.pm_intr_bufferfull, 1);
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PMCDBG6(SAM,INT,1,"(spc) cpu=%d pm=%p tf=%p um=%d wr=%d rd=%d",
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cpu, pm, (void *) tf, inuserspace,
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@ -4321,10 +4345,11 @@ pmc_process_samples(int cpu, int ring)
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if (pm == NULL || /* !cfg'ed */
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pm->pm_state != PMC_STATE_RUNNING || /* !active */
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!PMC_IS_SAMPLING_MODE(PMC_TO_MODE(pm)) || /* !sampling */
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pm->pm_stalled == 0) /* !stalled */
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!CPU_ISSET(cpu, &pm->pm_cpustate) || /* !desired */
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!CPU_ISSET(cpu, &pm->pm_stalled)) /* !stalled */
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continue;
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pm->pm_stalled = 0;
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CPU_CLR_ATOMIC(cpu, &pm->pm_stalled);
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(*pcd->pcd_start_pmc)(cpu, adjri);
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}
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}
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@ -4443,23 +4468,31 @@ pmc_process_exit(void *arg __unused, struct proc *p)
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("[pmc,%d] pm %p != pp_pmcs[%d] %p",
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__LINE__, pm, ri, pp->pp_pmcs[ri].pp_pmc));
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(void) pcd->pcd_stop_pmc(cpu, adjri);
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KASSERT(pm->pm_runcount > 0,
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("[pmc,%d] bad runcount ri %d rc %d",
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__LINE__, ri, pm->pm_runcount));
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/* Stop hardware only if it is actually running */
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if (pm->pm_state == PMC_STATE_RUNNING &&
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pm->pm_stalled == 0) {
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pcd->pcd_read_pmc(cpu, adjri, &newvalue);
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tmp = newvalue -
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PMC_PCPU_SAVED(cpu,ri);
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/*
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* Change desired state, and then stop if not
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* stalled. This two-step dance should avoid
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* race conditions where an interrupt re-enables
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* the PMC after this code has already checked
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* the pm_stalled flag.
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*/
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if (CPU_ISSET(cpu, &pm->pm_cpustate)) {
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CPU_CLR_ATOMIC(cpu, &pm->pm_cpustate);
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if (!CPU_ISSET(cpu, &pm->pm_stalled)) {
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(void) pcd->pcd_stop_pmc(cpu, adjri);
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pcd->pcd_read_pmc(cpu, adjri,
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&newvalue);
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tmp = newvalue -
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PMC_PCPU_SAVED(cpu,ri);
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mtx_pool_lock_spin(pmc_mtxpool, pm);
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pm->pm_gv.pm_savedvalue += tmp;
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pp->pp_pmcs[ri].pp_pmcval += tmp;
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mtx_pool_unlock_spin(pmc_mtxpool, pm);
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mtx_pool_lock_spin(pmc_mtxpool, pm);
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pm->pm_gv.pm_savedvalue += tmp;
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pp->pp_pmcs[ri].pp_pmcval += tmp;
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mtx_pool_unlock_spin(pmc_mtxpool, pm);
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}
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}
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atomic_subtract_rel_int(&pm->pm_runcount,1);
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@ -614,6 +614,7 @@ struct pmc_op_getdyneventinfo {
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#include <sys/malloc.h>
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#include <sys/sysctl.h>
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#include <sys/_cpuset.h>
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#include <machine/frame.h>
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@ -729,7 +730,8 @@ struct pmc {
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pmc_value_t pm_initial; /* counting PMC modes */
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} pm_sc;
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uint32_t pm_stalled; /* marks stalled sampling PMCs */
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volatile cpuset_t pm_stalled; /* marks stalled sampling PMCs */
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volatile cpuset_t pm_cpustate; /* CPUs where PMC should be active */
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uint32_t pm_caps; /* PMC capabilities */
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enum pmc_event pm_event; /* event being measured */
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uint32_t pm_flags; /* additional flags PMC_F_... */
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