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SubjectRe: [PATCH V6 1/1] Softirq:avoid large sched delay from the pending softirqs
On Wed, Sep 09, 2020 at 05:09:31PM +0800, qianjun.kernel@gmail.com wrote:
> From: jun qian <qianjun.kernel@gmail.com>
>
> When get the pending softirqs, it need to process all the pending
> softirqs in the while loop. If the processing time of each pending
> softirq is need more than 2 msec in this loop, or one of the softirq
> will running a long time, according to the original code logic, it
> will process all the pending softirqs without wakeuping ksoftirqd,
> which will cause a relatively large scheduling delay on the
> corresponding CPU, which we do not wish to see. The patch will check
> the total time to process pending softirq, if the time exceeds 2 ms
> we need to wakeup the ksofirqd to aviod large sched delay.

But what is all that unreadaable gibberish with pending_new_{flag,bit} ?

Random comments below..


> +#define MAX_SOFTIRQ_TIME_NS 2000000

2*NSEC_PER_MSEC


> +DEFINE_PER_CPU(__u32, pending_new_flag);
> +DEFINE_PER_CPU(__u32, pending_next_bit);

__u32 is for userspace ABI, this is not it, use u32

> +#define SOFTIRQ_PENDING_MASK ((1UL << NR_SOFTIRQS) - 1)
> +
> asmlinkage __visible void __softirq_entry __do_softirq(void)
> {
> - unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
> + u64 end = sched_clock() + MAX_SOFTIRQ_TIME_NS;
> unsigned long old_flags = current->flags;
> int max_restart = MAX_SOFTIRQ_RESTART;
> struct softirq_action *h;
> bool in_hardirq;
> - __u32 pending;
> - int softirq_bit;
> + __u32 pending, pending_left, pending_new;
> + int softirq_bit, next_bit;
> + unsigned long flags;
>
> /*
> * Mask out PF_MEMALLOC as the current task context is borrowed for the
> @@ -277,10 +282,33 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
>
> h = softirq_vec;
>
> - while ((softirq_bit = ffs(pending))) {
> - unsigned int vec_nr;
> + next_bit = per_cpu(pending_next_bit, smp_processor_id());
> + per_cpu(pending_new_flag, smp_processor_id()) = 0;

__this_cpu_read() / __this_cpu_write()

> +
> + pending_left = pending &
> + (SOFTIRQ_PENDING_MASK << next_bit);
> + pending_new = pending &
> + (SOFTIRQ_PENDING_MASK >> (NR_SOFTIRQS - next_bit));

The second mask is the inverse of the first.

> + /*
> + * In order to be fair, we shold process the pengding bits by the
> + * last processing order.
> + */
> + while ((softirq_bit = ffs(pending_left)) ||
> + (softirq_bit = ffs(pending_new))) {
> int prev_count;
> + unsigned int vec_nr = 0;
>
> + /*
> + * when the left pengding bits have been handled, we should
> + * to reset the h to softirq_vec.
> + */
> + if (!ffs(pending_left)) {
> + if (per_cpu(pending_new_flag, smp_processor_id()) == 0) {
> + h = softirq_vec;
> + per_cpu(pending_new_flag, smp_processor_id()) = 1;
> + }
> + }
> h += softirq_bit - 1;
>
> vec_nr = h - softirq_vec;
> @@ -298,17 +326,44 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
> preempt_count_set(prev_count);
> }
> h++;
> - pending >>= softirq_bit;
> +
> + if (ffs(pending_left))

This is the _third_ ffs(pending_left), those things are _expensive_ (on
some archs, see include/asm-generic/bitops/__ffs.h).

> + pending_left >>= softirq_bit;
> + else
> + pending_new >>= softirq_bit;
> +
> + /*
> + * the softirq's action has been run too much time,
> + * so it may need to wakeup the ksoftirqd
> + */
> + if (need_resched() && sched_clock() > end) {
> + /*
> + * Ensure that the remaining pending bits will be
> + * handled.
> + */
> + local_irq_save(flags);
> + if (ffs(pending_left))

*fourth*...

> + or_softirq_pending((pending_left << (vec_nr + 1)) |
> + pending_new);
> + else
> + or_softirq_pending(pending_new << (vec_nr + 1));
> + local_irq_restore(flags);
> + per_cpu(pending_next_bit, smp_processor_id()) = vec_nr + 1;
> + break;
> + }
> }
>
> + /* reset the pending_next_bit */
> + per_cpu(pending_next_bit, smp_processor_id()) = 0;
> +
> if (__this_cpu_read(ksoftirqd) == current)
> rcu_softirq_qs();
> local_irq_disable();
>
> pending = local_softirq_pending();
> if (pending) {
> - if (time_before(jiffies, end) && !need_resched() &&
> - --max_restart)
> + if (!need_resched() && --max_restart &&
> + sched_clock() <= end)
> goto restart;
>
> wakeup_softirqd();

This really wants to be a number of separate patches; and I quickly lost
the plot in your code. Instead of cleaning things up, you're making an
even bigger mess of things.

That said, I _think_ I've managed to decode what you want. See the
completely untested patches attached.


Subject: softirq: Rewrite softirq processing loop
From: Peter Zijlstra <peterz@infradead.org>
Date: Fri Sep 11 17:00:03 CEST 2020

Simplify the softirq processing loop by using the bitmap APIs

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/softirq.c | 16 ++++++----------
1 file changed, 6 insertions(+), 10 deletions(-)

--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -258,9 +258,9 @@ asmlinkage __visible void __softirq_entr
unsigned long old_flags = current->flags;
int max_restart = MAX_SOFTIRQ_RESTART;
struct softirq_action *h;
+ unsigned long pending;
+ unsigned int vec_nr;
bool in_hardirq;
- __u32 pending;
- int softirq_bit;

/*
* Mask out PF_MEMALLOC as the current task context is borrowed for the
@@ -281,15 +281,13 @@ asmlinkage __visible void __softirq_entr

local_irq_enable();

- h = softirq_vec;
+ for_each_set_bit(vec_nr, &pending, NR_SOFTIRQS) {
+ unsigned int prev_count;

- while ((softirq_bit = ffs(pending))) {
- unsigned int vec_nr;
- int prev_count;
+ __clear_bit(vec_nr, &pending);

- h += softirq_bit - 1;
+ h = softirq_vec + vec_nr;

- vec_nr = h - softirq_vec;
prev_count = preempt_count();

kstat_incr_softirqs_this_cpu(vec_nr);
@@ -303,8 +301,6 @@ asmlinkage __visible void __softirq_entr
prev_count, preempt_count());
preempt_count_set(prev_count);
}
- h++;
- pending >>= softirq_bit;
}

if (__this_cpu_read(ksoftirqd) == current)Subject: softirq: Use sched_clock() based timeout
From: Peter Zijlstra <peterz@infradead.org>
Date: Fri Sep 11 17:30:01 CEST 2020

Replace the jiffies based timeout with a sched_clock() based one.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/softirq.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -25,6 +25,7 @@
#include <linux/smpboot.h>
#include <linux/tick.h>
#include <linux/irq.h>
+#include <linux/sched/clock.h>

#define CREATE_TRACE_POINTS
#include <trace/events/irq.h>
@@ -216,7 +217,7 @@ EXPORT_SYMBOL(__local_bh_enable_ip);
* we want to handle softirqs as soon as possible, but they
* should not be able to lock up the box.
*/
-#define MAX_SOFTIRQ_TIME msecs_to_jiffies(2)
+#define MAX_SOFTIRQ_TIME 2*NSEC_PER_MSEC
#define MAX_SOFTIRQ_RESTART 10

#ifdef CONFIG_TRACE_IRQFLAGS
@@ -254,9 +255,9 @@ static inline void lockdep_softirq_end(b

asmlinkage __visible void __softirq_entry __do_softirq(void)
{
- unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
unsigned long old_flags = current->flags;
int max_restart = MAX_SOFTIRQ_RESTART;
+ u64 start = sched_clock();
struct softirq_action *h;
unsigned long pending;
unsigned int vec_nr;
@@ -309,7 +310,7 @@ asmlinkage __visible void __softirq_entr

pending = local_softirq_pending();
if (pending) {
- if (time_before(jiffies, end) && !need_resched() &&
+ if (sched_clock() - start < MAX_SOFTIRQ_TIME && !need_resched() &&
--max_restart)
goto restart;
Subject: softirq: Factor loop termination condition
From: Peter Zijlstra <peterz@infradead.org>
Date: Fri Sep 11 17:17:20 CEST 2020


Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/softirq.c | 44 +++++++++++++++++++++++++-------------------
1 file changed, 25 insertions(+), 19 deletions(-)

--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -204,22 +204,6 @@ void __local_bh_enable_ip(unsigned long
}
EXPORT_SYMBOL(__local_bh_enable_ip);

-/*
- * We restart softirq processing for at most MAX_SOFTIRQ_RESTART times,
- * but break the loop if need_resched() is set or after 2 ms.
- * The MAX_SOFTIRQ_TIME provides a nice upper bound in most cases, but in
- * certain cases, such as stop_machine(), jiffies may cease to
- * increment and so we need the MAX_SOFTIRQ_RESTART limit as
- * well to make sure we eventually return from this method.
- *
- * These limits have been established via experimentation.
- * The two things to balance is latency against fairness -
- * we want to handle softirqs as soon as possible, but they
- * should not be able to lock up the box.
- */
-#define MAX_SOFTIRQ_TIME 2*NSEC_PER_MSEC
-#define MAX_SOFTIRQ_RESTART 10
-
#ifdef CONFIG_TRACE_IRQFLAGS
/*
* When we run softirqs from irq_exit() and thus on the hardirq stack we need
@@ -253,10 +237,33 @@ static inline bool lockdep_softirq_start
static inline void lockdep_softirq_end(bool in_hardirq) { }
#endif

+/*
+ * We restart softirq processing but break the loop if need_resched() is set or
+ * after 2 ms. The MAX_SOFTIRQ_RESTART guarantees a loop termination if
+ * sched_clock() were ever to stall.
+ *
+ * These limits have been established via experimentation. The two things to
+ * balance is latency against fairness - we want to handle softirqs as soon as
+ * possible, but they should not be able to lock up the box.
+ */
+#define MAX_SOFTIRQ_TIME 2*NSEC_PER_MSEC
+#define MAX_SOFTIRQ_RESTART 10
+
+static inline bool __softirq_needs_break(u64 start)
+{
+ if (need_resched())
+ return true;
+
+ if (sched_clock() - start >= MAX_SOFTIRQ_TIME)
+ return true;
+
+ return false;
+}
+
asmlinkage __visible void __softirq_entry __do_softirq(void)
{
+ unsigned int max_restart = MAX_SOFTIRQ_RESTART;
unsigned long old_flags = current->flags;
- int max_restart = MAX_SOFTIRQ_RESTART;
u64 start = sched_clock();
struct softirq_action *h;
unsigned long pending;
@@ -310,8 +317,7 @@ asmlinkage __visible void __softirq_entr

pending = local_softirq_pending();
if (pending) {
- if (sched_clock() - start < MAX_SOFTIRQ_TIME && !need_resched() &&
- --max_restart)
+ if (!__softirq_needs_break(start) && --max_restart)
goto restart;

wakeup_softirqd();Subject: softirq: Allow early break
From: Peter Zijlstra <peterz@infradead.org>
Date: Fri Sep 11 17:50:17 CEST 2020

Allow terminating the softirq processing loop without finishing the
vectors.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/softirq.c | 16 ++++++++++------
1 file changed, 10 insertions(+), 6 deletions(-)

--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -309,19 +309,23 @@ asmlinkage __visible void __softirq_entr
prev_count, preempt_count());
preempt_count_set(prev_count);
}
+
+ if (pending && __softirq_needs_break(start))
+ break;
}

if (__this_cpu_read(ksoftirqd) == current)
rcu_softirq_qs();
local_irq_disable();

- pending = local_softirq_pending();
- if (pending) {
- if (!__softirq_needs_break(start) && --max_restart)
- goto restart;
+ if (pending)
+ or_softirq_pending(pending);
+ else if ((pending = local_softirq_pending()) &&
+ !__softirq_needs_break(start) &&
+ --max_restart)
+ goto restart;

- wakeup_softirqd();
- }
+ wakeup_softirqd();

lockdep_softirq_end(in_hardirq);
account_irq_exit_time(current);
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