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Message-ID: <168485357834.2849279.8073426325295894331.stgit@firesoul>
Date: Tue, 23 May 2023 16:52:58 +0200
From: Jesper Dangaard Brouer <brouer@...hat.com>
To: Ilias Apalodimas <ilias.apalodimas@...aro.org>, netdev@...r.kernel.org,
Eric Dumazet <eric.dumazet@...il.com>, linux-mm@...ck.org,
Mel Gorman <mgorman@...hsingularity.net>
Cc: Jesper Dangaard Brouer <brouer@...hat.com>, lorenzo@...nel.org,
Toke Høiland-Jørgensen <toke@...hat.com>,
linyunsheng@...wei.com, bpf@...r.kernel.org,
"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>,
willy@...radead.org
Subject: [PATCH RFC net-next/mm V4 2/2] page_pool: Remove workqueue in new
shutdown scheme
This removes the workqueue scheme that periodically tests when
inflight reach zero such that page_pool memory can be freed.
This change adds code to fast-path free checking for a shutdown flags
bit after returning PP pages.
Performance is very important for PP, as the fast path is used for
XDP_DROP use-cases where NIC drivers recycle PP pages directly into PP
alloc cache.
This patch (since V3) shows zero impact on this fast path. Micro
benchmarked with [1] on Intel CPU E5-1650 @3.60GHz. The slight code
reorg of likely() are deliberate.
[1] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/bench_page_pool_simple.c
Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
---
include/net/page_pool.h | 10 ++-
net/core/page_pool.c | 138 ++++++++++++++++++++++++++++++++++-------------
2 files changed, 104 insertions(+), 44 deletions(-)
diff --git a/include/net/page_pool.h b/include/net/page_pool.h
index c8ec2f34722b..19396e05f12d 100644
--- a/include/net/page_pool.h
+++ b/include/net/page_pool.h
@@ -50,6 +50,9 @@
PP_FLAG_DMA_SYNC_DEV |\
PP_FLAG_PAGE_FRAG)
+/* Internal flag: PP in shutdown phase, waiting for inflight pages */
+#define PP_FLAG_SHUTDOWN BIT(8)
+
/*
* Fast allocation side cache array/stack
*
@@ -151,11 +154,6 @@ static inline u64 *page_pool_ethtool_stats_get(u64 *data, void *stats)
struct page_pool {
struct page_pool_params p;
- struct delayed_work release_dw;
- void (*disconnect)(void *);
- unsigned long defer_start;
- unsigned long defer_warn;
-
u32 pages_state_hold_cnt;
unsigned int frag_offset;
struct page *frag_page;
@@ -165,6 +163,7 @@ struct page_pool {
/* these stats are incremented while in softirq context */
struct page_pool_alloc_stats alloc_stats;
#endif
+ void (*disconnect)(void *);
u32 xdp_mem_id;
/*
@@ -208,6 +207,7 @@ struct page_pool {
refcount_t user_cnt;
u64 destroy_cnt;
+ struct rcu_head rcu;
};
struct page *page_pool_alloc_pages(struct page_pool *pool, gfp_t gfp);
diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index e212e9d7edcb..213349148a48 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -23,9 +23,6 @@
#include <trace/events/page_pool.h>
-#define DEFER_TIME (msecs_to_jiffies(1000))
-#define DEFER_WARN_INTERVAL (60 * HZ)
-
#define BIAS_MAX LONG_MAX
#ifdef CONFIG_PAGE_POOL_STATS
@@ -380,6 +377,10 @@ static struct page *__page_pool_alloc_pages_slow(struct page_pool *pool,
struct page *page;
int i, nr_pages;
+ /* API usage BUG: PP in shutdown phase, cannot alloc new pages */
+ if (WARN_ON(pool->p.flags & PP_FLAG_SHUTDOWN))
+ return NULL;
+
/* Don't support bulk alloc for high-order pages */
if (unlikely(pp_order))
return __page_pool_alloc_page_order(pool, gfp);
@@ -450,9 +451,8 @@ EXPORT_SYMBOL(page_pool_alloc_pages);
*/
#define _distance(a, b) (s32)((a) - (b))
-static s32 page_pool_inflight(struct page_pool *pool)
+static s32 __page_pool_inflight(struct page_pool *pool, u32 release_cnt)
{
- u32 release_cnt = atomic_read(&pool->pages_state_release_cnt);
u32 hold_cnt = READ_ONCE(pool->pages_state_hold_cnt);
s32 inflight;
@@ -464,6 +464,14 @@ static s32 page_pool_inflight(struct page_pool *pool)
return inflight;
}
+static s32 page_pool_inflight(struct page_pool *pool)
+{
+ u32 release_cnt = atomic_read(&pool->pages_state_release_cnt);
+ return __page_pool_inflight(pool, release_cnt);
+}
+
+static int page_pool_free_attempt(struct page_pool *pool, u32 release_cnt);
+
/* Disconnects a page (from a page_pool). API users can have a need
* to disconnect a page (from a page_pool), to allow it to be used as
* a regular page (that will eventually be returned to the normal
@@ -472,7 +480,7 @@ static s32 page_pool_inflight(struct page_pool *pool)
void page_pool_release_page(struct page_pool *pool, struct page *page)
{
dma_addr_t dma;
- int count;
+ u32 release_cnt;
if (!(pool->p.flags & PP_FLAG_DMA_MAP))
/* Always account for inflight pages, even if we didn't
@@ -490,11 +498,12 @@ void page_pool_release_page(struct page_pool *pool, struct page *page)
skip_dma_unmap:
page_pool_clear_pp_info(page);
- /* This may be the last page returned, releasing the pool, so
- * it is not safe to reference pool afterwards.
- */
- count = atomic_inc_return_relaxed(&pool->pages_state_release_cnt);
- trace_page_pool_state_release(pool, page, count);
+ release_cnt = atomic_inc_return(&pool->pages_state_release_cnt);
+ trace_page_pool_state_release(pool, page, release_cnt);
+
+ /* In shutdown phase, last page will free pool instance */
+ if (READ_ONCE(pool->p.flags) & PP_FLAG_SHUTDOWN)
+ page_pool_free_attempt(pool, release_cnt);
}
EXPORT_SYMBOL(page_pool_release_page);
@@ -535,7 +544,7 @@ static bool page_pool_recycle_in_ring(struct page_pool *pool, struct page *page)
static bool page_pool_recycle_in_cache(struct page *page,
struct page_pool *pool)
{
- if (unlikely(pool->alloc.count == PP_ALLOC_CACHE_SIZE)) {
+ if (pool->alloc.count == PP_ALLOC_CACHE_SIZE) {
recycle_stat_inc(pool, cache_full);
return false;
}
@@ -546,6 +555,8 @@ static bool page_pool_recycle_in_cache(struct page *page,
return true;
}
+static void page_pool_empty_ring(struct page_pool *pool);
+
/* If the page refcnt == 1, this will try to recycle the page.
* if PP_FLAG_DMA_SYNC_DEV is set, we'll try to sync the DMA area for
* the configured size min(dma_sync_size, pool->max_len).
@@ -572,7 +583,8 @@ __page_pool_put_page(struct page_pool *pool, struct page *page,
page_pool_dma_sync_for_device(pool, page,
dma_sync_size);
- if (allow_direct && in_softirq() &&
+ /* During PP shutdown, no direct recycle must occur */
+ if (likely(allow_direct && in_softirq()) &&
page_pool_recycle_in_cache(page, pool))
return NULL;
@@ -609,6 +621,8 @@ void page_pool_put_defragged_page(struct page_pool *pool, struct page *page,
recycle_stat_inc(pool, ring_full);
page_pool_return_page(pool, page);
}
+ if (page && pool->p.flags & PP_FLAG_SHUTDOWN)
+ page_pool_empty_ring(pool);
}
EXPORT_SYMBOL(page_pool_put_defragged_page);
@@ -646,6 +660,9 @@ void page_pool_put_page_bulk(struct page_pool *pool, void **data,
recycle_stat_add(pool, ring, i);
page_pool_ring_unlock(pool);
+ if (pool->p.flags & PP_FLAG_SHUTDOWN)
+ page_pool_empty_ring(pool);
+
/* Hopefully all pages was return into ptr_ring */
if (likely(i == bulk_len))
return;
@@ -737,12 +754,18 @@ struct page *page_pool_alloc_frag(struct page_pool *pool,
}
EXPORT_SYMBOL(page_pool_alloc_frag);
+noinline
static void page_pool_empty_ring(struct page_pool *pool)
{
- struct page *page;
+ struct page *page, *next;
+
+ next = ptr_ring_consume_bh(&pool->ring);
/* Empty recycle ring */
- while ((page = ptr_ring_consume_bh(&pool->ring))) {
+ while (next) {
+ page = next;
+ next = ptr_ring_consume_bh(&pool->ring);
+
/* Verify the refcnt invariant of cached pages */
if (!(page_ref_count(page) == 1))
pr_crit("%s() page_pool refcnt %d violation\n",
@@ -768,6 +791,14 @@ static void page_pool_free(struct page_pool *pool)
kfree(pool);
}
+static void page_pool_free_rcu(struct rcu_head *rcu)
+{
+ struct page_pool *pool;
+
+ pool = container_of(rcu, struct page_pool, rcu);
+ page_pool_free(pool);
+}
+
static void page_pool_empty_alloc_cache_once(struct page_pool *pool)
{
struct page *page;
@@ -796,39 +827,30 @@ static void page_pool_scrub(struct page_pool *pool)
page_pool_empty_ring(pool);
}
-static int page_pool_release(struct page_pool *pool)
+noinline
+static int page_pool_free_attempt(struct page_pool *pool, u32 release_cnt)
{
int inflight;
- page_pool_scrub(pool);
- inflight = page_pool_inflight(pool);
+ rcu_read_lock();
+ inflight = __page_pool_inflight(pool, release_cnt);
+ rcu_read_unlock();
if (!inflight)
- page_pool_free(pool);
+ call_rcu(&pool->rcu, page_pool_free_rcu);
return inflight;
}
-static void page_pool_release_retry(struct work_struct *wq)
+static int page_pool_release(struct page_pool *pool)
{
- struct delayed_work *dwq = to_delayed_work(wq);
- struct page_pool *pool = container_of(dwq, typeof(*pool), release_dw);
int inflight;
- inflight = page_pool_release(pool);
+ page_pool_scrub(pool);
+ inflight = page_pool_inflight(pool);
if (!inflight)
- return;
-
- /* Periodic warning */
- if (time_after_eq(jiffies, pool->defer_warn)) {
- int sec = (s32)((u32)jiffies - (u32)pool->defer_start) / HZ;
-
- pr_warn("%s() stalled pool shutdown %d inflight %d sec\n",
- __func__, inflight, sec);
- pool->defer_warn = jiffies + DEFER_WARN_INTERVAL;
- }
+ page_pool_free(pool);
- /* Still not ready to be disconnected, retry later */
- schedule_delayed_work(&pool->release_dw, DEFER_TIME);
+ return inflight;
}
void page_pool_use_xdp_mem(struct page_pool *pool, void (*disconnect)(void *),
@@ -856,6 +878,10 @@ EXPORT_SYMBOL(page_pool_unlink_napi);
void page_pool_destroy(struct page_pool *pool)
{
+ unsigned int flags;
+ u32 release_cnt;
+ u32 hold_cnt;
+
if (!pool)
return;
@@ -868,11 +894,45 @@ void page_pool_destroy(struct page_pool *pool)
if (!page_pool_release(pool))
return;
- pool->defer_start = jiffies;
- pool->defer_warn = jiffies + DEFER_WARN_INTERVAL;
+ /* PP have pages inflight, thus cannot immediately release memory.
+ * Enter into shutdown phase, depending on remaining in-flight PP
+ * pages to trigger shutdown process (on concurrent CPUs) and last
+ * page will free pool instance.
+ *
+ * There exist two race conditions here, we need to take into
+ * account in the following code.
+ *
+ * 1. Before setting PP_FLAG_SHUTDOWN another CPU released the last
+ * pages into the ptr_ring. Thus, it missed triggering shutdown
+ * process, which can then be stalled forever.
+ *
+ * 2. After setting PP_FLAG_SHUTDOWN another CPU released the last
+ * page, which triggered shutdown process and freed pool
+ * instance. Thus, its not safe to dereference *pool afterwards.
+ *
+ * Handling races by holding a fake in-flight count, via artificially
+ * bumping pages_state_hold_cnt, which assures pool isn't freed under
+ * us. Use RCU Grace-Periods to guarantee concurrent CPUs will
+ * transition safely into the shutdown phase.
+ *
+ * After safely transition into this state the races are resolved. For
+ * race(1) its safe to recheck and empty ptr_ring (it will not free
+ * pool). Race(2) cannot happen, and we can release fake in-flight count
+ * as last step.
+ */
+ hold_cnt = READ_ONCE(pool->pages_state_hold_cnt) + 1;
+ WRITE_ONCE(pool->pages_state_hold_cnt, hold_cnt);
+ synchronize_rcu();
+
+ flags = READ_ONCE(pool->p.flags) | PP_FLAG_SHUTDOWN;
+ WRITE_ONCE(pool->p.flags, flags);
+ synchronize_rcu();
+
+ /* Concurrent CPUs could have returned last pages into ptr_ring */
+ page_pool_empty_ring(pool);
- INIT_DELAYED_WORK(&pool->release_dw, page_pool_release_retry);
- schedule_delayed_work(&pool->release_dw, DEFER_TIME);
+ release_cnt = atomic_inc_return(&pool->pages_state_release_cnt);
+ page_pool_free_attempt(pool, release_cnt);
}
EXPORT_SYMBOL(page_pool_destroy);
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