On Wed, Feb 12, 2020 at 6:56 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, 27 Jan 2020 18:59:58 -0800 Arjun Roy <arjunroy.kdev@xxxxxxxxx> wrote:
>
> > Use vm_insert_pages() for tcp receive zerocopy. Spin lock cycles
> > (as reported by perf) drop from a couple of percentage points
> > to a fraction of a percent. This results in a roughly 6% increase in
> > efficiency, measured roughly as zerocopy receive count divided by CPU
> > utilization.
> >
> > The intention of this patch-set is to reduce atomic ops for
> > tcp zerocopy receives, which normally hits the same spinlock multiple
> > times consecutively.
>
> For some reason the patch causes this:
>
> In file included from ./arch/x86/include/asm/atomic.h:5:0,
> from ./include/linux/atomic.h:7,
> from ./include/linux/crypto.h:15,
> from ./include/crypto/hash.h:11,
> from net/ipv4/tcp.c:246:
> net/ipv4/tcp.c: In function ‘do_tcp_getsockopt.isra.29’:
> ./include/linux/compiler.h:225:31: warning: ‘tp’ may be used uninitialized in this function [-Wmaybe-uninitialized]
> case 4: *(volatile __u32 *)p = *(__u32 *)res; break;
> ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
> net/ipv4/tcp.c:1779:19: note: ‘tp’ was declared here
> struct tcp_sock *tp;
> ^~
>
> It's a false positive. gcc-7.2.0
>
> : out:
> : up_read(¤t->mm->mmap_sem);
> : if (length) {
> : WRITE_ONCE(tp->copied_seq, seq);
>
> but `length' is zero here.
>
> This suppresses it:
>
> --- a/net/ipv4/tcp.c~net-zerocopy-use-vm_insert_pages-for-tcp-rcv-zerocopy-fix
> +++ a/net/ipv4/tcp.c
> @@ -1788,6 +1788,8 @@ static int tcp_zerocopy_receive(struct s
>
> sock_rps_record_flow(sk);
>
> + tp = tcp_sk(sk);
> +
> down_read(¤t->mm->mmap_sem);
>
> ret = -EINVAL;
> @@ -1796,7 +1798,6 @@ static int tcp_zerocopy_receive(struct s
> goto out;
> zc->length = min_t(unsigned long, zc->length, vma->vm_end - address);
>
> - tp = tcp_sk(sk);
> seq = tp->copied_seq;
> inq = tcp_inq(sk);
> zc->length = min_t(u32, zc->length, inq);
>
> and I guess it's zero-cost.
>
>
> Anyway, I'll sit on this lot for a while, hoping for a davem ack?
Actually, speaking of the ack on the networking side:
I guess this patch set is a bit weird since it requires some
non-trivial coordination between mm and net-next? Not sure what the
normal approach is in this case.
-Arjun
