On Tue, Nov 08, 2022 at 10:41:53AM -0800, Song Liu wrote: > On Tue, Nov 8, 2022 at 3:27 AM Mike Rapoport <rppt@xxxxxxxxxx> 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; Unless I'm missing something, what will be shared is the virtual space, the actual physical pages will be still allocated the same way as any vmalloc() allocation. > 3) There are bigger BPF programs in many use cases. With statistics you provided above one will need hundreds if not thousands of BPF programs to fill a 2M page. I didn't do the math, but it seems that to see memory savings there should be several hundreds of BPF programs. > Thanks, > Song -- Sincerely yours, Mike.
