iv/linux
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

powerpc/64s/interrupt: Don't enable MSR[EE] in irq handlers unless perf is in use

Browse Source 
Enabling MSR[EE] in interrupt handlers while interrupts are still soft
masked allows PMIs to profile interrupt handlers to some degree, beyond
what SIAR latching allows.

When perf is not being used, this is almost useless work. It requires an
extra mtmsrd in the irq handler, and it also opens the door to masked
interrupts hitting and requiring replay, which is more expensive than
just taking them directly. This effect can be noticable in high IRQ
workloads.

Avoid enabling MSR[EE] unless perf is currently in use. This saves about
60 cycles (or 8%) on a simple decrementer interrupt microbenchmark.
Replayed interrupts drop from 1.4% of all interrupts taken, to 0.003%.

This does prevent the soft-nmi interrupt being taken in these handlers,
but that's not too reliable anyway. The SMP watchdog will continue to be
the reliable way to catch lockups.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210922145452.352571-5-npiggin@gmail.com
This commit is contained in:
Nicholas Piggin 2021-09-23 00:54:50 +10:00 committed by Michael Ellerman
parent 5a7745b96f
commit 0faf20a1ad
4 changed files with 67 additions and 27 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

57
arch/powerpc/include/asm/hw_irq.h
View File

@ -345,17 +345,54 @@ static inline bool lazy_irq_pending_nocheck(void)
bool power_pmu_wants_prompt_pmi(void);
/*
* This is called by asynchronous interrupts to conditionally
* re-enable hard interrupts after having cleared the source
* of the interrupt. They are kept disabled if there is a different
* soft-masked interrupt pending that requires hard masking.
* This is called by asynchronous interrupts to check whether to
* conditionally re-enable hard interrupts after having cleared
* the source of the interrupt. They are kept disabled if there
* is a different soft-masked interrupt pending that requires hard
* masking.
*/
static inline void may_hard_irq_enable(void)
static inline bool should_hard_irq_enable(void)
{
if (!(get_paca()->irq_happened & PACA_IRQ_MUST_HARD_MASK)) {
get_paca()->irq_happened &= ~PACA_IRQ_HARD_DIS;
__hard_irq_enable();
}
#ifdef CONFIG_PPC_IRQ_SOFT_MASK_DEBUG
WARN_ON(irq_soft_mask_return() == IRQS_ENABLED);
WARN_ON(mfmsr() & MSR_EE);
#endif
#ifdef CONFIG_PERF_EVENTS
/*
* If the PMU is not running, there is not much reason to enable
* MSR[EE] in irq handlers because any interrupts would just be
* soft-masked.
*
* TODO: Add test for 64e
*/
if (IS_ENABLED(CONFIG_PPC_BOOK3S_64) && !power_pmu_wants_prompt_pmi())
return false;
if (get_paca()->irq_happened & PACA_IRQ_MUST_HARD_MASK)
return false;
return true;
#else
return false;
#endif
}
/*
* Do the hard enabling, only call this if should_hard_irq_enable is true.
*/
static inline void do_hard_irq_enable(void)
{
#ifdef CONFIG_PPC_IRQ_SOFT_MASK_DEBUG
WARN_ON(irq_soft_mask_return() == IRQS_ENABLED);
WARN_ON(get_paca()->irq_happened & PACA_IRQ_MUST_HARD_MASK);
WARN_ON(mfmsr() & MSR_EE);
#endif
/*
* This allows PMI interrupts (and watchdog soft-NMIs) through.
* There is no other reason to enable this way.
*/
get_paca()->irq_happened &= ~PACA_IRQ_HARD_DIS;
__hard_irq_enable();
}
static inline bool arch_irq_disabled_regs(struct pt_regs *regs)
@ -436,7 +473,7 @@ static inline bool arch_irq_disabled_regs(struct pt_regs *regs)
return !(regs->msr & MSR_EE);
}
static inline bool may_hard_irq_enable(void)
static inline bool should_hard_irq_enable(void)
{
return false;
}

3
arch/powerpc/kernel/dbell.c
View File

@ -27,7 +27,8 @@ DEFINE_INTERRUPT_HANDLER_ASYNC(doorbell_exception)
ppc_msgsync();
may_hard_irq_enable();
if (should_hard_irq_enable())
do_hard_irq_enable();
kvmppc_clear_host_ipi(smp_processor_id());
__this_cpu_inc(irq_stat.doorbell_irqs);

3
arch/powerpc/kernel/irq.c
View File

@ -745,7 +745,8 @@ void __do_irq(struct pt_regs *regs)
irq = ppc_md.get_irq();
/* We can hard enable interrupts now to allow perf interrupts */
may_hard_irq_enable();
if (should_hard_irq_enable())
do_hard_irq_enable();
/* And finally process it */
if (unlikely(!irq))

31
arch/powerpc/kernel/time.c
View File

@ -609,22 +609,23 @@ DEFINE_INTERRUPT_HANDLER_ASYNC(timer_interrupt)
return;
}
/* Ensure a positive value is written to the decrementer, or else
* some CPUs will continue to take decrementer exceptions. When the
* PPC_WATCHDOG (decrementer based) is configured, keep this at most
* 31 bits, which is about 4 seconds on most systems, which gives
* the watchdog a chance of catching timer interrupt hard lockups.
*/
if (IS_ENABLED(CONFIG_PPC_WATCHDOG))
set_dec(0x7fffffff);
else
set_dec(decrementer_max);
/* Conditionally hard-enable interrupts now that the DEC has been
* bumped to its maximum value
*/
may_hard_irq_enable();
/* Conditionally hard-enable interrupts. */
if (should_hard_irq_enable()) {
/*
* Ensure a positive value is written to the decrementer, or
* else some CPUs will continue to take decrementer exceptions.
* When the PPC_WATCHDOG (decrementer based) is configured,
* keep this at most 31 bits, which is about 4 seconds on most
* systems, which gives the watchdog a chance of catching timer
* interrupt hard lockups.
*/
if (IS_ENABLED(CONFIG_PPC_WATCHDOG))
set_dec(0x7fffffff);
else
set_dec(decrementer_max);
do_hard_irq_enable();
}
#if defined(CONFIG_PPC32) && defined(CONFIG_PPC_PMAC)
if (atomic_read(&ppc_n_lost_interrupts) != 0)