[net-next,v2] tcp: Use per-vma locking for receive zerocopy

Commit Message

Arjun Roy June 16, 2023, 7:34 p.m. UTC

From: Arjun Roy <[e-mail]>

Per-VMA locking allows us to lock a struct vm_area_struct without
taking the process-wide mmap lock in read mode.

Consider a process workload where the mmap lock is taken constantly in
write mode. In this scenario, all zerocopy receives are periodically
blocked during that period of time - though in principle, the memory
ranges being used by TCP are not touched by the operations that need
the mmap write lock. This results in performance degradation.

Now consider another workload where the mmap lock is never taken in
write mode, but there are many TCP connections using receive zerocopy
that are concurrently receiving. These connections all take the mmap
lock in read mode, but this does induce a lot of contention and atomic
ops for this process-wide lock. This results in additional CPU
overhead caused by contending on the cache line for this lock.

However, with per-vma locking, both of these problems can be avoided.

As a test, I ran an RPC-style request/response workload with 4KB
payloads and receive zerocopy enabled, with 100 simultaneous TCP
connections. I measured perf cycles within the
find_tcp_vma/mmap_read_lock/mmap_read_unlock codepath, with and
without per-vma locking enabled.

When using process-wide mmap semaphore read locking, about 1% of
measured perf cycles were within this path. With per-VMA locking, this
value dropped to about 0.45%.

Signed-off-by: Arjun Roy <[e-mail]>
Reviewed-by: Eric Dumazet <[e-mail]>
---
v2 change:
Fixed linker error on builds without CONFIG_INET.
---
 MAINTAINERS            |  1 +
 include/linux/net_mm.h | 17 ++++++++++++++++
 include/net/tcp.h      |  1 +
 mm/memory.c            |  7 ++++---
 net/ipv4/tcp.c         | 45 ++++++++++++++++++++++++++++++++++--------
 5 files changed, 60 insertions(+), 11 deletions(-)
 create mode 100644 include/linux/net_mm.h

Comments

[e-mail] June 18, 2023, 10:20 a.m. UTC | #1

Hello:

This patch was applied to netdev/net-next.git (main)
by David S. Miller <[e-mail]>:

On Fri, 16 Jun 2023 12:34:27 -0700 you wrote:
> From: Arjun Roy <[e-mail]>
> 
> Per-VMA locking allows us to lock a struct vm_area_struct without
> taking the process-wide mmap lock in read mode.
> 
> Consider a process workload where the mmap lock is taken constantly in
> write mode. In this scenario, all zerocopy receives are periodically
> blocked during that period of time - though in principle, the memory
> ranges being used by TCP are not touched by the operations that need
> the mmap write lock. This results in performance degradation.
> 
> [...]

Here is the summary with links:
  - [net-next,v2] tcp: Use per-vma locking for receive zerocopy
    https://git.kernel.org/netdev/net-next/c/7a7f09463534

You are awesome, thank you!

Matthew Wilcox July 6, 2023, 7:43 p.m. UTC | #2

On Fri, Jun 16, 2023 at 12:34:27PM -0700, Arjun Roy wrote:
> However, with per-vma locking, both of these problems can be avoided.

I appreciate your enthusiasm for this.  However, applying this patch
completely wrecks my patch series to push per-vma locking down for
file-backed mappings.  It would be helpful if we can back this out
for now and then apply it after that patch series.

Would it make life hard for this patch to go through the mm tree?

Eric Dumazet July 7, 2023, 3:44 p.m. UTC | #3

On Thu, Jul 6, 2023 at 9:43 PM Matthew Wilcox <[e-mail]> wrote:
>
> On Fri, Jun 16, 2023 at 12:34:27PM -0700, Arjun Roy wrote:
> > However, with per-vma locking, both of these problems can be avoided.
>
> I appreciate your enthusiasm for this.  However, applying this patch
> completely wrecks my patch series to push per-vma locking down for
> file-backed mappings.  It would be helpful if we can back this out
> for now and then apply it after that patch series.
>
> Would it make life hard for this patch to go through the mm tree?
>

No worries, can you send a formal revert then ?

(With some details, because I do not see why you can not simply add
the revert in front of your series ?)

Thanks.

Patch

diff --git a/MAINTAINERS b/MAINTAINERS
index c6fa6ed454f4..a7c495e3323b 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -14727,6 +14727,7 @@  NETWORKING [TCP]
 M:	Eric Dumazet <[e-mail]>
 L:	[e-mail]
 S:	Maintained
+F:	include/linux/net_mm.h
 F:	include/linux/tcp.h
 F:	include/net/tcp.h
 F:	include/trace/events/tcp.h
diff --git a/include/linux/net_mm.h b/include/linux/net_mm.h
new file mode 100644
index 000000000000..b298998bd5a0
--- /dev/null
+++ b/include/linux/net_mm.h
@@ -0,0 +1,17 @@ 
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+#ifdef CONFIG_MMU
+
+#ifdef CONFIG_INET
+extern const struct vm_operations_struct tcp_vm_ops;
+static inline bool vma_is_tcp(const struct vm_area_struct *vma)
+{
+	return vma->vm_ops == &tcp_vm_ops;
+}
+#else
+static inline bool vma_is_tcp(const struct vm_area_struct *vma)
+{
+	return false;
+}
+#endif /* CONFIG_INET*/
+
+#endif /* CONFIG_MMU */
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 5066e4586cf0..bfa5e27205ba 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -45,6 +45,7 @@ 
 #include <linux/memcontrol.h>
 #include <linux/bpf-cgroup.h>
 #include <linux/siphash.h>
+#include <linux/net_mm.h>
 
 extern struct inet_hashinfo tcp_hashinfo;
 
diff --git a/mm/memory.c b/mm/memory.c
index f69fbc251198..3e46b4d881dc 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -77,6 +77,7 @@ 
 #include <linux/ptrace.h>
 #include <linux/vmalloc.h>
 #include <linux/sched/sysctl.h>
+#include <linux/net_mm.h>
 
 #include <trace/events/kmem.h>
 
@@ -5280,12 +5281,12 @@  struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
 	if (!vma)
 		goto inval;
 
-	/* Only anonymous vmas are supported for now */
-	if (!vma_is_anonymous(vma))
+	/* Only anonymous and tcp vmas are supported for now */
+	if (!vma_is_anonymous(vma) && !vma_is_tcp(vma))
 		goto inval;
 
 	/* find_mergeable_anon_vma uses adjacent vmas which are not locked */
-	if (!vma->anon_vma)
+	if (!vma->anon_vma && !vma_is_tcp(vma))
 		goto inval;
 
 	if (!vma_start_read(vma))
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 8d20d9221238..6240d81476b8 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1877,7 +1877,7 @@  void tcp_update_recv_tstamps(struct sk_buff *skb,
 }
 
 #ifdef CONFIG_MMU
-static const struct vm_operations_struct tcp_vm_ops = {
+const struct vm_operations_struct tcp_vm_ops = {
 };
 
 int tcp_mmap(struct file *file, struct socket *sock,
@@ -2176,6 +2176,34 @@  static void tcp_zc_finalize_rx_tstamp(struct sock *sk,
 	}
 }
 
+static struct vm_area_struct *find_tcp_vma(struct mm_struct *mm,
+					   unsigned long address,
+					   bool *mmap_locked)
+{
+	struct vm_area_struct *vma = NULL;
+
+#ifdef CONFIG_PER_VMA_LOCK
+	vma = lock_vma_under_rcu(mm, address);
+#endif
+	if (vma) {
+		if (!vma_is_tcp(vma)) {
+			vma_end_read(vma);
+			return NULL;
+		}
+		*mmap_locked = false;
+		return vma;
+	}
+
+	mmap_read_lock(mm);
+	vma = vma_lookup(mm, address);
+	if (!vma || !vma_is_tcp(vma)) {
+		mmap_read_unlock(mm);
+		return NULL;
+	}
+	*mmap_locked = true;
+	return vma;
+}
+
 #define TCP_ZEROCOPY_PAGE_BATCH_SIZE 32
 static int tcp_zerocopy_receive(struct sock *sk,
 				struct tcp_zerocopy_receive *zc,
@@ -2193,6 +2221,7 @@  static int tcp_zerocopy_receive(struct sock *sk,
 	u32 seq = tp->copied_seq;
 	u32 total_bytes_to_map;
 	int inq = tcp_inq(sk);
+	bool mmap_locked;
 	int ret;
 
 	zc->copybuf_len = 0;
@@ -2217,13 +2246,10 @@  static int tcp_zerocopy_receive(struct sock *sk,
 		return 0;
 	}
 
-	mmap_read_lock(current->mm);
-
-	vma = vma_lookup(current->mm, address);
-	if (!vma || vma->vm_ops != &tcp_vm_ops) {
-		mmap_read_unlock(current->mm);
+	vma = find_tcp_vma(current->mm, address, &mmap_locked);
+	if (!vma)
 		return -EINVAL;
-	}
+
 	vma_len = min_t(unsigned long, zc->length, vma->vm_end - address);
 	avail_len = min_t(u32, vma_len, inq);
 	total_bytes_to_map = avail_len & ~(PAGE_SIZE - 1);
@@ -2297,7 +2323,10 @@  static int tcp_zerocopy_receive(struct sock *sk,
 						   zc, total_bytes_to_map);
 	}
 out:
-	mmap_read_unlock(current->mm);
+	if (mmap_locked)
+		mmap_read_unlock(current->mm);
+	else
+		vma_end_read(vma);
 	/* Try to copy straggler data. */
 	if (!ret)
 		copylen = tcp_zc_handle_leftover(zc, sk, skb, &seq, copybuf_len, tss);