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; 3) There are bigger BPF programs in many use cases. > > > Based on our experiments [5], we measured 0.5% performance improvement > > from bpf_prog_pack. This patchset further boosts the improvement to 0.7%. > > 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. > > The proposed execmem_alloc() looks to me very much tailored for x86 to be > used as a replacement for module_alloc(). Some architectures have > module_alloc() that is quite different from the default or x86 version, so > I'd expect at least some explanation how modules etc can use execmem_ APIs > without breaking !x86 architectures. > > > This set enables bpf programs and bpf dispatchers to share huge pages with > > new API: > > execmem_alloc() > > execmem_alloc() > > execmem_fill() > > > > The idea is similar to Peter's suggestion in [1]. > > > > execmem_alloc() manages a set of PMD_SIZE RO+X memory, and allocates these > > memory to its users. execmem_alloc() is used to free memory allocated by > > execmem_alloc(). execmem_fill() is used to update memory allocated by > > execmem_alloc(). > > > > Memory allocated by execmem_alloc() is RO+X, so this doesnot violate W^X. > > The caller has to update the content with text_poke like mechanism. > > Specifically, execmem_fill() is provided to update memory allocated by > > execmem_alloc(). execmem_fill() also makes sure the update stays in the > > boundary of one chunk allocated by execmem_alloc(). Please refer to patch > > 1/5 for more details of > > Unless I'm mistaken, a failure to allocate PMD_SIZE page will fail text > allocation altogether. That means that if somebody tries to load a BFP > program on a busy long lived system, they are quite likely to fail because > high order free lists might be already exhausted although there is still > plenty of free memory. > > Did you consider a fallback for small pages if the high order allocation > fails? I think __vmalloc_node_range() already has the fallback mechanism. (the end of the function). Thanks, Song
