On Thu, Jul 09, 2020 at 04:55:57PM -0700, paul...@kernel.org wrote:
So the biggest problem I have with this patch is that while it adds a
Kconfig debug knob, it doesn't seem to actually use it.
Stuff like:
> +/* Record current CSD work for current CPU, NULL to erase. */
> +static void csd_lock_record(call_single_data_t *csd)
> +{
> + if (!csd) {
> + smp_mb(); // NULL cur_csd after unlock.
> + __this_cpu_write(cur_csd, NULL);
> + return;
> + }
> + __this_cpu_write(cur_csd, csd);
> + __this_cpu_write(cur_csd_func, csd->func);
> + __this_cpu_write(cur_csd_info, csd->info);
> + smp_mb(); // Update cur_csd before function call.
> + // Or before unlock, as the case may be.
> +}
> +
> +static __always_inline int csd_lock_wait_getcpu(call_single_data_t *csd)
> +{
> + unsigned int csd_type;
> +
> + csd_type = CSD_TYPE(csd);
> +#ifdef CONFIG_64BIT
> + if (csd_type == CSD_TYPE_ASYNC || csd_type == CSD_TYPE_SYNC)
> + return csd->dst; // Other CSD_TYPE_ values might not have ->dst.
> +#endif
> + return -1;
> +}
is unconditional, and thus adds unconditional fulll barriers :-(
> +/*
> + * Complain if too much time spent waiting. Note that only
> + * the CSD_TYPE_SYNC/ASYNC types provide the destination CPU,
> + * so waiting on other types gets much less information.
> + */
> +static __always_inline bool csd_lock_wait_toolong(call_single_data_t *csd,
> u64 ts0, u64 *ts1, int *bug_id)
> +{
> + int cpu = -1;
> + call_single_data_t *cpu_cur_csd;
> + bool firsttime;
> + unsigned int flags = READ_ONCE(csd->flags);
> + u64 quo;
> + u32 rem;
> + u64 ts2, ts_delta;
> +
> + if (!(flags & CSD_FLAG_LOCK)) {
> + if (!unlikely(*bug_id))
> + return true;
> + cpu = csd_lock_wait_getcpu(csd);
> + if (cpu >= 0)
> + pr_alert("csd: CSD lock (#%d) got unstuck on CPU#%02d,
> CPU#%02d released the lock after all. Phew!\n", *bug_id,
> raw_smp_processor_id(), cpu);
> + else
> + pr_alert("csd: CSD lock (#%d) got unstuck on CPU#%02d,
> the lock was released after all. Phew!\n", *bug_id, raw_smp_processor_id());
> + return true;
> + }
> +
> + ts2 = div_u64_rem(sched_clock(), 1000 * 1000, &rem);
> + ts_delta = ts2 - *ts1;
> + if (likely(ts_delta <= CSD_LOCK_TIMEOUT)) {
> + cpu_relax();
> + return false;
> + }
> +
> + firsttime = !*bug_id;
> + if (firsttime)
> + *bug_id = atomic_inc_return(&csd_bug_count);
> + cpu = csd_lock_wait_getcpu(csd);
> + smp_mb(); // No stale cur_csd values!
> + if (WARN_ONCE(cpu < 0 || cpu >= nr_cpu_ids, "%s: cpu = %d\n", __func__,
> cpu))
> + cpu_cur_csd = READ_ONCE(per_cpu(cur_csd, 0));
> + else
> + cpu_cur_csd = READ_ONCE(per_cpu(cur_csd, cpu));
> + smp_mb(); // No refetching cur_csd values!
> + quo = div_u64_rem(ts2 - ts0, 1000, &rem);
> +#define CSD_FORMAT_PREFIX "csd: %s non-responsive CSD lock (#%d) on CPU#%d,
> waiting %llu.%03u secs for CPU#%02d %pS(%ps), currently"
> +#define CSD_ARGS_PREFIX firsttime ? "Detected" : "Continued", *bug_id,
> raw_smp_processor_id(), \
> + quo, rem, cpu, csd->func, csd->info
> + if (cpu_cur_csd && csd != cpu_cur_csd)
> + pr_alert(CSD_FORMAT_PREFIX " handling prior %pS(%ps)
> request.\n",
> + CSD_ARGS_PREFIX, cpu_cur_csd->func, cpu_cur_csd->info);
> + else
> + pr_alert(CSD_FORMAT_PREFIX " %s.\n", CSD_ARGS_PREFIX,
> + !cpu_cur_csd ? "unresponsive" : "handling this
> request");
> +#undef CSD_FORMAT_PREFIX
> +#undef CSD_ARGS_PREFIX
> + if (cpu >= 0) {
> + if (!trigger_single_cpu_backtrace(cpu))
> + dump_cpu_task(cpu);
> + if (!cpu_cur_csd) {
> + pr_alert("csd: Re-sending CSD lock (#%d) IPI from
> CPU#%02d to CPU#%02d\n", *bug_id, raw_smp_processor_id(), cpu);
> + arch_send_call_function_single_ipi(cpu);
> + }
> + }
> + dump_stack();
> + *ts1 = ts2;
> + cpu_relax();
> +
> + return false;
> +}
This thing is just unreadable. Did it want to be something like:
bool warn = false;
for (;;) {
while (sched_clock() - t1 < NSEC_PER_MSEC * CSD_TIMO) {
if (!(READ_ONCE(csd->flags) & CSD_FLAG_LOCK))
goto out;
}
t1 += NSEC_PER_MSEC * CSD_TIMO;
pr_alert("csd: stuck on %d, waiting on %d\n", csd->src,
csd->dst);
if (!warn) {
warn = true;
WARN_ON(1);
}
arch_send_call_function_single_ipi(csd->dst);
}
out:
smp_acquire__after_ctrl_dep();
if (warn)
pr_alert("csd: got unstuck..\n");
> +
> /*
> * csd_lock/csd_unlock used to serialize access to per-cpu csd resources
> *
> @@ -105,7 +212,19 @@ void __init call_function_init(void)
> */
> static __always_inline void csd_lock_wait(call_single_data_t *csd)
> {
> - smp_cond_load_acquire(&csd->flags, !(VAL & CSD_FLAG_LOCK));
> + int bug_id = 0;
> + u32 rem;
> + u64 ts0, ts1;
> +
> + if (!IS_ENABLED(CONFIG_CSD_LOCK_WAIT_DEBUG)) {
> + smp_cond_load_acquire(&csd->flags, !(VAL & CSD_FLAG_LOCK));
> + return;
> + }
> + ts1 = ts0 = div_u64_rem(sched_clock(), 1000 * 1000, &rem);
> + for (;;)
> + if (csd_lock_wait_toolong(csd, ts0, &ts1, &bug_id))
> + break;
> + smp_acquire__after_ctrl_dep();
> }
This makes a trainwreck from what was a very simple small function.
I'm thinking you can get rid of that division and simply multiply things
by a 1e6 to get a timeout in ns, that'll be _waaaaay_ faster on
everything that doesn't have an oversized divider.
Anyway, what's wrong with the age old way of:
#ifdef CONFIG_CSD_LOCK_WAIT_DEBUG
static inline void csd_lock_record(call_single_data_t *csd)
{
/* extra barriers go here */
}
static void csd_lock_wait(call_single_data *csd)
{
/* giant monster goes here */
}
#else
static __always_inline void csd_lock_record(call_single_data_t *csd)
{
}
static __always_inline void csd_lock_wait(call_single_data_t *csd)
{
smp_cond_load_acquire(&csd->flags, !(VAL & CSD_FLAG_LOCK));
}
#endif /* CONFIG_CSD_LOCK_WAIT_DEBUG */
> @@ -375,7 +500,10 @@ int smp_call_function_single(int cpu, smp_call_func_t
> func, void *info,
>
> csd->func = func;
> csd->info = info;
> +#ifdef CONFIG_64BIT
> + csd->src = smp_processor_id();
> csd->dst = cpu;
> +#endif
>
> err = generic_exec_single(cpu, csd);
>
> @@ -541,7 +669,10 @@ static void smp_call_function_many_cond(const struct
> cpumask *mask,
> csd->flags |= CSD_TYPE_SYNC;
> csd->func = func;
> csd->info = info;
> +#ifdef CONFIG_64BIT
> + csd->src = smp_processor_id();
> csd->dst = cpu;
> +#endif
> if (llist_add(&csd->llist, &per_cpu(call_single_queue, cpu)))
> __cpumask_set_cpu(cpu, cfd->cpumask_ipi);
> }
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 669f4d3..22443fa3 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1332,6 +1332,16 @@ config SCF_TORTURE_TEST
> module may be built after the fact on the running kernel to
> be tested, if desired.
>
> +config CSD_LOCK_WAIT_DEBUG
> + bool "Debugging for csd_lock_wait(), called from smp_call_function*()"
> + depends on DEBUG_KERNEL
Would it make sense to put the src,dst members under this config
variable too and make this whole thing
depends on CONFIG_64BIT
?
> + default n
> + help
> + This option enables debug prints when CPUs are slow to respond
> + to the smp_call_function*() IPI wrappers. These debug prints
> + include the IPI handler function currently executing (if any)
> + and relevant stack traces.
> +
> endmenu # lock debugging
>
> config TRACE_IRQFLAGS
> --
> 2.9.5
>
