Messages in this thread | | | From | Eric Dumazet <> | Date | Wed, 10 Nov 2021 09:43:39 -0800 | Subject | Re: [PATCH 2/2] jump_label: refine placement of static_keys |
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On Wed, Nov 10, 2021 at 9:06 AM Ard Biesheuvel <ardb@kernel.org> wrote: > > On Wed, 10 Nov 2021 at 16:22, Eric Dumazet <edumazet@google.com> wrote: > > > > On Wed, Nov 10, 2021 at 2:24 AM Ard Biesheuvel <ardb@kernel.org> wrote: > > > > > > On Wed, 10 Nov 2021 at 09:36, Peter Zijlstra <peterz@infradead.org> wrote: > > > > > > > > On Tue, Nov 09, 2021 at 05:09:06PM -0800, Eric Dumazet wrote: > > > > > From: Eric Dumazet <edumazet@google.com> > > > > > > > > > > With CONFIG_JUMP_LABEL=y, "struct static_key" content is only > > > > > used for the control path. > > > > > > > > > > Marking them __read_mostly is only needed when CONFIG_JUMP_LABEL=n. > > > > > Otherwise we place them out of the way to increase data locality. > > > > > > > > > > This patch adds __static_key to centralize this new policy. > > > > > > > > > > Signed-off-by: Eric Dumazet <edumazet@google.com> > > > > > --- > > > > > arch/x86/kvm/lapic.c | 4 ++-- > > > > > arch/x86/kvm/x86.c | 2 +- > > > > > include/linux/jump_label.h | 25 +++++++++++++++++-------- > > > > > kernel/events/core.c | 2 +- > > > > > kernel/sched/fair.c | 2 +- > > > > > net/core/dev.c | 8 ++++---- > > > > > net/netfilter/core.c | 2 +- > > > > > net/netfilter/x_tables.c | 2 +- > > > > > 8 files changed, 28 insertions(+), 19 deletions(-) > > > > > > > > > > > > > Hurmph, it's a bit cumbersome to always have to add this __static_key > > > > attribute to every definition, and in fact you seem to have missed some. > > > > > > > > Would something like: > > > > > > > > typedef struct static_key __static_key static_key_t; > > > > > > > > work? I forever seem to forget the exact things you can make a typedef > > > > do :/ > > > > > > No, that doesn't work. Section placement is an attribute of the symbol > > > not of its type. So we'll need to macro'ify this. > > > > Yes, this is also why I chose a short __static_key (initially I was > > using something more descriptive but longer) > > > > > > > > But I'm not sure I understand why we need different policies here. > > > Static keys are inherently __read_mostly (unless they are not writable > > > to begin with), so keeping them all together in one place in the > > > binary should be sufficient, no? > > > > It is not optimal for CONFIG_JUMP_LABEL=n cases. > > > > For instance, networking will prefer having rps_needed / rfs_needed in > > the same cache lines than other hot read_mostly stuff, > > instead of being far away in other locations. > > > > ffffffff830e0f80 D dev_weight_tx_bias > > ffffffff830e0f84 D dev_rx_weight > > ffffffff830e0f88 D dev_tx_weight > > ffffffff830e0f8c D gro_normal_batch > > ffffffff830e0f90 D rps_sock_flow_table > > ffffffff830e0f98 D rps_cpu_mask > > ffffffff830e0f9c D rps_needed > > ffffffff830e0fa0 D rfs_needed > > ffffffff830e0fa4 D netdev_flow_limit_table_len > > ffffffff830e0fa8 d netif_napi_add.__print_once > > ffffffff830e0fac D netdev_unregister_timeout_secs > > ffffffff830e0fb0 D ptype_base > > > > > > When CONFIG_JUMP_LABEL=y, rps_needed/xps_needed being in a remote > > location is a win because it 'saves' 32 bytes than can be used better > > I understand that you want the key out of the way for > CONFIG_JUMP_LABEL=n, but the question was why we shouldn't do that > unconditionally. If we put all the keys together in a section, they > will only share cachelines with each other. > > Also, what is the performance impact on a real world use case of this change?
Yes, this matters for low latency stuff, mostly.
For CONFIG_JUMP_LABEL=n, I suggest we do not change the current layout, there is no need to. I do not want to risk performance regressions for no good reason.
Unless you have something in mind _requiring_ all these atomic_t being grouped together ?
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