On Mon, Nov 21, 2022 at 1:12 PM Luis Chamberlain <mcgrof@xxxxxxxxxx> wrote: > > On Thu, Nov 17, 2022 at 12:23:16PM -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. > > This value is clear, but I'd like to see at least another new user and > the respective commit log show the gains as Aaron Lu showed. > > > 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. > > As suggested by Mike Rapoport, "benchmarking iTLB performance on an idle > system is not very representative. TLB is a scarce resource, so it'd be > interesting to see this benchmark on a loaded system." > > This would also help pave the way to measure this for more possible > future callers like modules. There in lies true value to this > consideration. > > Also, you mention your perf stats are run on a VM, I am curious what > things you need to get TLB to be properly measured on the VM and if > this is really reliable data Vs bare metal. I haven't yet been sucessful > on getting perf stat for TBL to work on a VM and based on what I've read > have been catious about the results. To make these perf counters work on VM, we need a newer host kernel (my system is running 5.6 based kernel, but I am not sure what is the minimum required version). Then we need to run qemu with option "-cpu host" (both host and guest are x86_64). > > So curious if you'd see something different on bare metal. Once the above all worked out, VM runs the same as bare metal from perf counter's point of view. > > [0] https://lkml.kernel.org/r/Y3YA2mRZDJkB4lmP@xxxxxxxxxx > > > 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. > > If this is all done on the bpf code replacement then the commit log > should clarify that in the commit log, as then it allows future users > to not be surprised if they don't see these gains as this is specific > to the way bpf code used bpf_prog_pag. Also, you can measure the > shootdowns and show the differences with perf stat tlb: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. > > > > 5. Introduce a unified API to allocate memory with special permissions. > > > > This will help get rid of set_vm_flush_reset_perms calls from users of > > vmalloc, module_alloc, etc. > > And *this* is one of the reasons I'm so eager to see a proper solution > drawn up. This would be a huge win for modules, however since some of > the complexities in special permissions with modules lies in all the > cross architecture hanky panky, I'd prefer to see this through merged > *iff* we have modules converted as well as it would give us a clearer > picture if the solution covers the bases. And we'd get proper testing > on this. Rather than it being a special thing for BPF. > > > Based on our experiments [5], we measured ~0.6% performance improvement > > from bpf_prog_pack. This patchset further boosts the improvement to ~0.8%. > > I'd prefer we leave out arbitrary performance data, as it does not help much. This really bothers me. With real workload, we are talking about performance difference of ~1%. I don't think there is any open source benchmark that can show this level of performance difference. In our case, we used A/B test with 80 hosts (40 vs. 40) and runs for many hours to confidently show 1% performance difference. This exact benchmark has a very good record of reporting smallish performance regression. For example, this commit commit 7af0145067bc ("x86/mm/cpa: Avoid the 4k pages check completely") fixes a bug that splits the page table (from 2MB to 4kB) for the WHOLE kernel text. The bug stayed in the kernel for almost a year. None of all the available open source benchmark had caught it before this specific benchmark. We have used this benchmark to demonstrate performance benefits of many optimizations. I don't understand why it suddenly becomes "arbitrary performance data". Song > > > The difference is because bpf_prog_pack uses 512x 4kB pages instead of > > 1x 2MB page, bpf_prog_pack as-is doesn't resolve #2 above. > > > > This patchset replaces bpf_prog_pack with a better API and makes it > > available for other dynamic kernel text, such as modules, ftrace, kprobe. > > Let's see that through, then I think the series builds confidence in > implementation.
