Re: Re: [PATCH v3 4/5] sock: Consider memcg pressure when raising sockmem

[Abel Wu]

On 5/23/23 6:26 PM, Paolo Abeni wrote:
> On Tue, 2023-05-23 at 17:46 +0800, Abel Wu wrote:
> > For now __sk_mem_raise_allocated() mainly considers global socket
> > memory pressure and allows to raise if no global pressure observed,
> > including the sockets whose memcgs are in pressure, which might
> > result in longer memcg memstall.
> >
> > So take net-memcg's pressure into consideration when allocating
> > socket memory to alleviate long tail latencies.
> >
> > Signed-off-by: Abel Wu <wuyun.abel@xxxxxxxxxxxxx>
> > ---
> >   net/core/sock.c | 23 ++++++++++++++++-------
> >   1 file changed, 16 insertions(+), 7 deletions(-)
> >
> > diff --git a/net/core/sock.c b/net/core/sock.c
> > index 801df091e37a..b899e0b9feda 100644
> > --- a/net/core/sock.c
> > +++ b/net/core/sock.c
> > @@ -2976,22 +2976,31 @@ EXPORT_SYMBOL(sk_wait_data);
> >   int __sk_mem_raise_allocated(struct sock *sk, int size, int amt, int kind)
> >   {
> >   	bool memcg_charge = mem_cgroup_sockets_enabled && sk->sk_memcg;
> > +	bool charged = true, pressured = false;
> >   	struct proto *prot = sk->sk_prot;
> > -	bool charged = true;
> >   	long allocated;
> >   
> >   	sk_memory_allocated_add(sk, amt);
> >   	allocated = sk_memory_allocated(sk);
> > -	if (memcg_charge &&
> > -	    !(charged = mem_cgroup_charge_skmem(sk->sk_memcg, amt,
> > -						gfp_memcg_charge())))
> > -		goto suppress_allocation;
> > +
> > +	if (memcg_charge) {
> > +		charged = mem_cgroup_charge_skmem(sk->sk_memcg, amt,
> > +						  gfp_memcg_charge());
> > +		if (!charged)
> > +			goto suppress_allocation;
> > +		if (mem_cgroup_under_socket_pressure(sk->sk_memcg))
> > +			pressured = true;
> > +	}
> >   
> >   	/* Under limit. */
> > -	if (allocated <= sk_prot_mem_limits(sk, 0)) {
> > +	if (allocated <= sk_prot_mem_limits(sk, 0))
> >   		sk_leave_memory_pressure(sk);
> > +	else
> > +		pressured = true;
>
> The above looks not correct to me.
>
> 	allocated > sk_prot_mem_limits(sk, 0)
>
> does not mean the protocol has memory pressure. Such condition is
> checked later with:
>
> 	if (allocated > sk_prot_mem_limits(sk, 1))

Yes, this condition stands means the global socket memory is absolutely
under pressure, and the status is sustained until @allocated falls down
to sk_prot_mem_limits(sk, 0). I see some places in the source tree call
it 'soft pressure' if usage between index [0] and [1].

The idea behind this patch is to allow the socket memory to raise if
there is no pressure neither in global nor net-memcg. With the condition

	@allocated > sk_prot_mem_limits(sk, 0)

we can't be sure whether there is pressure or not in global. And this
also aligns with the original logic if net-memcg is not used.

I am thinking changing the name of this variable to @might_pressured or
something to better illustrate the status of memory pressure. What do
you think?

> Here an allocation could fail even if memcg charge is successful and
> the protocol is not under pressure, which in turn sounds quite (too
> much?) conservative.

IIUC the failure can only be due to its memcg under vmpressure. In this
case allowing the allocation would burden the mm subsys with increased
fragmented unmovable/unreclaimable memory.

Thanks & Best,
	Abel