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Subject[RFC][PATCH 5/7] irq_work, smp: Allow irq_work on call_single_queue
Currently irq_work_queue_on() will issue an unconditional
arch_send_call_function_single_ipi() and has the handler do
irq_work_run().

This is unfortunate in that it makes the IPI handler look at a second
cacheline and it misses the opportunity to avoid the IPI. Instead note
that struct irq_work and struct __call_single_data are very similar in
layout, so use a few bits in the flags word to encode a type and stick
the irq_work on the call_single_queue list.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/irq_work.h | 7 ++
include/linux/smp.h | 23 ++++++++-
kernel/irq_work.c | 53 +++++++++++---------
kernel/smp.c | 119 ++++++++++++++++++++++++++++-------------------
4 files changed, 130 insertions(+), 72 deletions(-)

--- a/include/linux/irq_work.h
+++ b/include/linux/irq_work.h
@@ -13,6 +13,8 @@
* busy NULL, 2 -> {free, claimed} : callback in progress, can be claimed
*/

+/* flags share CSD_FLAG_ space */
+
#define IRQ_WORK_PENDING BIT(0)
#define IRQ_WORK_BUSY BIT(1)

@@ -23,9 +25,12 @@

#define IRQ_WORK_CLAIMED (IRQ_WORK_PENDING | IRQ_WORK_BUSY)

+/*
+ * structure shares layout with single_call_data_t.
+ */
struct irq_work {
- atomic_t flags;
struct llist_node llnode;
+ atomic_t flags;
void (*func)(struct irq_work *);
};

--- a/include/linux/smp.h
+++ b/include/linux/smp.h
@@ -16,17 +16,38 @@

typedef void (*smp_call_func_t)(void *info);
typedef bool (*smp_cond_func_t)(int cpu, void *info);
+
+enum {
+ CSD_FLAG_LOCK = 0x01,
+
+ /* IRQ_WORK_flags */
+
+ CSD_TYPE_ASYNC = 0x00,
+ CSD_TYPE_SYNC = 0x10,
+ CSD_TYPE_IRQ_WORK = 0x20,
+ CSD_FLAG_TYPE_MASK = 0xF0,
+};
+
+/*
+ * structure shares (partial) layout with struct irq_work
+ */
struct __call_single_data {
struct llist_node llist;
+ unsigned int flags;
smp_call_func_t func;
void *info;
- unsigned int flags;
};

/* Use __aligned() to avoid to use 2 cache lines for 1 csd */
typedef struct __call_single_data call_single_data_t
__aligned(sizeof(struct __call_single_data));

+/*
+ * Enqueue a llist_node on the call_single_queue; be very careful, read
+ * flush_smp_call_function_queue() in detail.
+ */
+extern void __smp_call_single_queue(int cpu, struct llist_node *node);
+
/* total number of cpus in this system (may exceed NR_CPUS) */
extern unsigned int total_cpus;

--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -31,7 +31,7 @@ static bool irq_work_claim(struct irq_wo
{
int oflags;

- oflags = atomic_fetch_or(IRQ_WORK_CLAIMED, &work->flags);
+ oflags = atomic_fetch_or(IRQ_WORK_CLAIMED | CSD_TYPE_IRQ_WORK, &work->flags);
/*
* If the work is already pending, no need to raise the IPI.
* The pairing atomic_fetch_andnot() in irq_work_run() makes sure
@@ -102,8 +102,7 @@ bool irq_work_queue_on(struct irq_work *
if (cpu != smp_processor_id()) {
/* Arch remote IPI send/receive backend aren't NMI safe */
WARN_ON_ONCE(in_nmi());
- if (llist_add(&work->llnode, &per_cpu(raised_list, cpu)))
- arch_send_call_function_single_ipi(cpu);
+ __smp_call_single_queue(cpu, &work->llnode);
} else {
__irq_work_queue_local(work);
}
@@ -131,6 +130,31 @@ bool irq_work_needs_cpu(void)
return true;
}

+void irq_work_single(void *arg)
+{
+ struct irq_work *work = arg;
+ int flags;
+
+ /*
+ * Clear the PENDING bit, after this point the @work
+ * can be re-used.
+ * Make it immediately visible so that other CPUs trying
+ * to claim that work don't rely on us to handle their data
+ * while we are in the middle of the func.
+ */
+ flags = atomic_fetch_andnot(IRQ_WORK_PENDING, &work->flags);
+
+ lockdep_irq_work_enter(work);
+ work->func(work);
+ lockdep_irq_work_exit(work);
+ /*
+ * Clear the BUSY bit and return to the free state if
+ * no-one else claimed it meanwhile.
+ */
+ flags &= ~IRQ_WORK_PENDING;
+ (void)atomic_cmpxchg(&work->flags, flags, flags & ~IRQ_WORK_BUSY);
+}
+
static void irq_work_run_list(struct llist_head *list)
{
struct irq_work *work, *tmp;
@@ -142,27 +166,8 @@ static void irq_work_run_list(struct lli
return;

llnode = llist_del_all(list);
- llist_for_each_entry_safe(work, tmp, llnode, llnode) {
- int flags;
- /*
- * Clear the PENDING bit, after this point the @work
- * can be re-used.
- * Make it immediately visible so that other CPUs trying
- * to claim that work don't rely on us to handle their data
- * while we are in the middle of the func.
- */
- flags = atomic_fetch_andnot(IRQ_WORK_PENDING, &work->flags);
-
- lockdep_irq_work_enter(work);
- work->func(work);
- lockdep_irq_work_exit(work);
- /*
- * Clear the BUSY bit and return to the free state if
- * no-one else claimed it meanwhile.
- */
- flags &= ~IRQ_WORK_PENDING;
- (void)atomic_cmpxchg(&work->flags, flags, flags & ~IRQ_WORK_BUSY);
- }
+ llist_for_each_entry_safe(work, tmp, llnode, llnode)
+ irq_work_single(work);
}

/*
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -23,10 +23,8 @@

#include "smpboot.h"

-enum {
- CSD_FLAG_LOCK = 0x01,
- CSD_FLAG_SYNCHRONOUS = 0x02,
-};
+
+#define CSD_TYPE(_csd) ((_csd)->flags & CSD_FLAG_TYPE_MASK)

struct call_function_data {
call_single_data_t __percpu *csd;
@@ -137,15 +135,33 @@ static DEFINE_PER_CPU_SHARED_ALIGNED(cal

extern void send_call_function_single_ipi(int cpu);

+void __smp_call_single_queue(int cpu, struct llist_node *node)
+{
+ /*
+ * The list addition should be visible before sending the IPI
+ * handler locks the list to pull the entry off it because of
+ * normal cache coherency rules implied by spinlocks.
+ *
+ * If IPIs can go out of order to the cache coherency protocol
+ * in an architecture, sufficient synchronisation should be added
+ * to arch code to make it appear to obey cache coherency WRT
+ * locking and barrier primitives. Generic code isn't really
+ * equipped to do the right thing...
+ */
+ if (llist_add(node, &per_cpu(call_single_queue, cpu)))
+ send_call_function_single_ipi(cpu);
+}
+
/*
* Insert a previously allocated call_single_data_t element
* for execution on the given CPU. data must already have
* ->func, ->info, and ->flags set.
*/
-static int generic_exec_single(int cpu, call_single_data_t *csd,
- smp_call_func_t func, void *info)
+static int generic_exec_single(int cpu, call_single_data_t *csd)
{
if (cpu == smp_processor_id()) {
+ smp_call_func_t func = csd->func;
+ void *info = csd->info;
unsigned long flags;

/*
@@ -159,28 +175,12 @@ static int generic_exec_single(int cpu,
return 0;
}

-
if ((unsigned)cpu >= nr_cpu_ids || !cpu_online(cpu)) {
csd_unlock(csd);
return -ENXIO;
}

- csd->func = func;
- csd->info = info;
-
- /*
- * The list addition should be visible before sending the IPI
- * handler locks the list to pull the entry off it because of
- * normal cache coherency rules implied by spinlocks.
- *
- * If IPIs can go out of order to the cache coherency protocol
- * in an architecture, sufficient synchronisation should be added
- * to arch code to make it appear to obey cache coherency WRT
- * locking and barrier primitives. Generic code isn't really
- * equipped to do the right thing...
- */
- if (llist_add(&csd->llist, &per_cpu(call_single_queue, cpu)))
- send_call_function_single_ipi(cpu);
+ __smp_call_single_queue(cpu, &csd->llist);

return 0;
}
@@ -194,16 +194,10 @@ static int generic_exec_single(int cpu,
void generic_smp_call_function_single_interrupt(void)
{
flush_smp_call_function_queue(true);
-
- /*
- * Handle irq works queued remotely by irq_work_queue_on().
- * Smp functions above are typically synchronous so they
- * better run first since some other CPUs may be busy waiting
- * for them.
- */
- irq_work_run();
}

+extern void irq_work_single(void *);
+
/**
* flush_smp_call_function_queue - Flush pending smp-call-function callbacks
*
@@ -241,9 +235,21 @@ static void flush_smp_call_function_queu
* We don't have to use the _safe() variant here
* because we are not invoking the IPI handlers yet.
*/
- llist_for_each_entry(csd, entry, llist)
- pr_warn("IPI callback %pS sent to offline CPU\n",
- csd->func);
+ llist_for_each_entry(csd, entry, llist) {
+ switch (CSD_TYPE(csd)) {
+ case CSD_TYPE_ASYNC:
+ case CSD_TYPE_SYNC:
+ case CSD_TYPE_IRQ_WORK:
+ pr_warn("IPI callback %pS sent to offline CPU\n",
+ csd->func);
+ break;
+
+ default:
+ pr_warn("IPI callback, unknown type %d, sent to offline CPU\n",
+ CSD_TYPE(csd));
+ break;
+ }
+ }
}

/*
@@ -251,16 +257,17 @@ static void flush_smp_call_function_queu
*/
prev = NULL;
llist_for_each_entry_safe(csd, csd_next, entry, llist) {
- smp_call_func_t func = csd->func;
- void *info = csd->info;
-
/* Do we wait until *after* callback? */
- if (csd->flags & CSD_FLAG_SYNCHRONOUS) {
+ if (CSD_TYPE(csd) == CSD_TYPE_SYNC) {
+ smp_call_func_t func = csd->func;
+ void *info = csd->info;
+
if (prev) {
prev->next = &csd_next->llist;
} else {
entry = &csd_next->llist;
}
+
func(info);
csd_unlock(csd);
} else {
@@ -272,11 +279,17 @@ static void flush_smp_call_function_queu
* Second; run all !SYNC callbacks.
*/
llist_for_each_entry_safe(csd, csd_next, entry, llist) {
- smp_call_func_t func = csd->func;
- void *info = csd->info;
+ int type = CSD_TYPE(csd);

- csd_unlock(csd);
- func(info);
+ if (type == CSD_TYPE_ASYNC) {
+ smp_call_func_t func = csd->func;
+ void *info = csd->info;
+
+ csd_unlock(csd);
+ func(info);
+ } else if (type == CSD_TYPE_IRQ_WORK) {
+ irq_work_single(csd);
+ }
}
}

@@ -305,7 +318,7 @@ int smp_call_function_single(int cpu, sm
{
call_single_data_t *csd;
call_single_data_t csd_stack = {
- .flags = CSD_FLAG_LOCK | CSD_FLAG_SYNCHRONOUS,
+ .flags = CSD_FLAG_LOCK | CSD_TYPE_SYNC,
};
int this_cpu;
int err;
@@ -339,7 +352,10 @@ int smp_call_function_single(int cpu, sm
csd_lock(csd);
}

- err = generic_exec_single(cpu, csd, func, info);
+ csd->func = func;
+ csd->info = info;
+
+ err = generic_exec_single(cpu, csd);

if (wait)
csd_lock_wait(csd);
@@ -385,7 +401,7 @@ int smp_call_function_single_async(int c
csd->flags = CSD_FLAG_LOCK;
smp_wmb();

- err = generic_exec_single(cpu, csd, csd->func, csd->info);
+ err = generic_exec_single(cpu, csd);

out:
preempt_enable();
@@ -500,7 +516,7 @@ static void smp_call_function_many_cond(

csd_lock(csd);
if (wait)
- csd->flags |= CSD_FLAG_SYNCHRONOUS;
+ csd->flags |= CSD_TYPE_SYNC;
csd->func = func;
csd->info = info;
if (llist_add(&csd->llist, &per_cpu(call_single_queue, cpu)))
@@ -632,6 +648,17 @@ void __init smp_init(void)
{
int num_nodes, num_cpus;

+ /*
+ * Ensure struct irq_work layout matches so that
+ * flush_smp_call_function_queue() can do horrible things.
+ */
+ BUILD_BUG_ON(offsetof(struct irq_work, llnode) !=
+ offsetof(struct __call_single_data, llist));
+ BUILD_BUG_ON(offsetof(struct irq_work, func) !=
+ offsetof(struct __call_single_data, func));
+ BUILD_BUG_ON(offsetof(struct irq_work, flags) !=
+ offsetof(struct __call_single_data, flags));
+
idle_threads_init();
cpuhp_threads_init();


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