From 8df5a500a3e97f7811cdce0f553ca1917ccd4220 Mon Sep 17 00:00:00 2001 From: Stephane Eranian Date: Mon, 11 Apr 2005 13:45:00 -0700 Subject: [IA64] perfmon & PAL_HALT again The pmu_active test is based on the values of PSR.up. THIS IS THE PROBLEM as it does not take into account the lazy restore logic which is as follow (simplified): context switch out: save PMDs clear psr.up release ownership context switch in: if (ctx->last_cpu == smp_processor_id() && ctx->cpu_activation == cpu_activation) { set psr.up return } restore PMD restore PMC ctx->last_cpu = smp_processor_id(); ctx->activation = ++cpu_activation; set psr.up The key here is that on context switch out, we clear psr.up and on context switch in we check if nobody else used the PMU on that processor since last time we came. In that case, we assume the PMD/PMC are ours and we simply reactivate. The Caliper problem is that between the moment we context switch out and the moment we come back, nobody effectively used the PMU BUT the processor went idle. Normally this would have no incidence but PAL_HALT does alter the PMU registers. In default_idle(), the test on psr.up is not strong enough to cover this case and we go into PAL which trashed the PMU resgisters. When we come back we falsely assume that this is our state yet it is corrupted. Very nasty indeed. To avoid the problem it is necessary to forbid going to PAL_HALT as soon as perfmon installs some valid state in the PMU registers. This happens with an application attaches a context to a thread or CPU. It is not enough to check the psr/dcr bits. Hence I propose the attached patch. It adds a callback in process.c to modify the condition to enter PAL on idle. Basically, now it is conditional to pal_halt=1 AND perfmon saying it is okay. Signed-off-by: Tony Luck --- arch/ia64/kernel/perfmon.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) (limited to 'arch/ia64/kernel/perfmon.c') diff --git a/arch/ia64/kernel/perfmon.c b/arch/ia64/kernel/perfmon.c index 376fcbc3f8da..fd4f3be6e856 100644 --- a/arch/ia64/kernel/perfmon.c +++ b/arch/ia64/kernel/perfmon.c @@ -1265,6 +1265,8 @@ out: } EXPORT_SYMBOL(pfm_unregister_buffer_fmt); +extern void update_pal_halt_status(int); + static int pfm_reserve_session(struct task_struct *task, int is_syswide, unsigned int cpu) { @@ -1311,6 +1313,11 @@ pfm_reserve_session(struct task_struct *task, int is_syswide, unsigned int cpu) is_syswide, cpu)); + /* + * disable default_idle() to go to PAL_HALT + */ + update_pal_halt_status(0); + UNLOCK_PFS(flags); return 0; @@ -1366,6 +1373,12 @@ pfm_unreserve_session(pfm_context_t *ctx, int is_syswide, unsigned int cpu) is_syswide, cpu)); + /* + * if possible, enable default_idle() to go into PAL_HALT + */ + if (pfm_sessions.pfs_task_sessions == 0 && pfm_sessions.pfs_sys_sessions == 0) + update_pal_halt_status(1); + UNLOCK_PFS(flags); return 0; -- cgit v1.2.1