Re: [PATCH bpf-next v2 0/5] execmem_alloc for BPF programs

[Song Liu <song@kernel.org>]

On Tue, Nov 8, 2022 at 11:43 AM Christophe Leroy
<christophe.leroy@csgroup.eu> wrote:
>
>
>
> Le 08/11/2022 à 19:41, Song Liu a écrit :
> > On Tue, Nov 8, 2022 at 3:27 AM Mike Rapoport <rppt@kernel.org> wrote:
> >>
> >> Hi Song,
> >>
> >> On Mon, Nov 07, 2022 at 02:39:16PM -0800, Song Liu wrote:
> >>> This patchset tries to address the following issues:
> >>>
> >>> 1. Direct map fragmentation
> >>>
> >>> On x86, STRICT_*_RWX requires the direct map of any RO+X memory to be also
> >>> RO+X. These set_memory_* calls cause 1GB page table entries to be split
> >>> into 2MB and 4kB ones. This fragmentation in direct map results in bigger
> >>> and slower page table, and pressure for both instruction and data TLB.
> >>>
> >>> Our previous work in bpf_prog_pack tries to address this issue from BPF
> >>> program side. Based on the experiments by Aaron Lu [4], bpf_prog_pack has
> >>> greatly reduced direct map fragmentation from BPF programs.
> >>
> >> Usage of set_memory_* APIs with memory allocated from vmalloc/modules
> >> virtual range does not change the direct map, but only updates the
> >> permissions in vmalloc range. The direct map splits occur in
> >> vm_remove_mappings() when the memory is *freed*.
> >>
> >> That said, both bpf_prog_pack and these patches do reduce the
> >> fragmentation, but this happens because the memory is freed to the system
> >> in 2M chunks and there are no splits of 2M pages. Besides, since the same
> >> 2M page used for many BPF programs there should be way less vfree() calls.
> >>
> >>> 2. iTLB pressure from BPF program
> >>>
> >>> Dynamic kernel text such as modules and BPF programs (even with current
> >>> bpf_prog_pack) use 4kB pages on x86, when the total size of modules and
> >>> BPF program is big, we can see visible performance drop caused by high
> >>> iTLB miss rate.
> >>
> >> Like Luis mentioned several times already, it would be nice to see numbers.
> >>
> >>> 3. TLB shootdown for short-living BPF programs
> >>>
> >>> Before bpf_prog_pack loading and unloading BPF programs requires global
> >>> TLB shootdown. This patchset (and bpf_prog_pack) replaces it with a local
> >>> TLB flush.
> >>>
> >>> 4. Reduce memory usage by BPF programs (in some cases)
> >>>
> >>> Most BPF programs and various trampolines are small, and they often
> >>> occupies a whole page. From a random server in our fleet, 50% of the
> >>> loaded BPF programs are less than 500 byte in size, and 75% of them are
> >>> less than 2kB in size. Allowing these BPF programs to share 2MB pages
> >>> would yield some memory saving for systems with many BPF programs. For
> >>> systems with only small number of BPF programs, this patch may waste a
> >>> little memory by allocating one 2MB page, but using only part of it.
> >>
> >> I'm not convinced there are memory savings here. Unless you have hundreds
> >> of BPF programs, most of 2M page will be wasted, won't it?
> >> So for systems that have moderate use of BPF most of the 2M page will be
> >> unused, right?
> >
> > There will be some memory waste in such cases. But it will get better with:
> > 1) With 4/5 and 5/5, BPF programs will share this 2MB page with kernel .text
> > section (_stext to _etext);
> > 2) modules, ftrace, kprobe will also share this 2MB page;
> > 3) There are bigger BPF programs in many use cases.
>
> And what I love with this series (for powerpc/32) is that we will likely
> now be able to have bpf, ftrace, kprobe without the performance cost of
> CONFIG_MODULES.

Yeah, I remember reading emails about using tracing tools without
CONFIG_MODULES. We still need more work (beyond this set) to make it
happen for powerpc/32. For example, current powerpc bpf_jit doesn't
support jitting into ROX memory.

Song


>
> Today, CONFIG_MODULES means page mapping, which means handling of kernel
> page in ITLB miss handlers.
>
> By using some of the space between end of rodata and start of inittext,
> we are able to use ROX linear memory which is mapped by blocks. It means
> there is no need to handle kernel text in ITLB handler (You can look at
> https://elixir.bootlin.com/linux/v6.1-rc3/source/arch/powerpc/kernel/head_8xx.S#L191
> to better understand what I'm talking about).
>
> Thanks
> Christophe