On Tue, May 28, 2019 at 08:29:47AM +0200, Michal Hocko wrote: > On Tue 28-05-19 12:26:32, Minchan Kim wrote: > > On Mon, May 27, 2019 at 02:44:11PM +0200, Michal Hocko wrote: > > > On Mon 27-05-19 16:58:11, Minchan Kim wrote: > > > > On Tue, May 21, 2019 at 08:26:28AM +0200, Michal Hocko wrote: > > > > > On Tue 21-05-19 11:55:33, Minchan Kim wrote: > > > > > > On Mon, May 20, 2019 at 11:28:01AM +0200, Michal Hocko wrote: > > > > > > > [cc linux-api] > > > > > > > > > > > > > > On Mon 20-05-19 12:52:54, Minchan Kim wrote: > > > > > > > > System could have much faster swap device like zRAM. In that case, swapping > > > > > > > > is extremely cheaper than file-IO on the low-end storage. > > > > > > > > In this configuration, userspace could handle different strategy for each > > > > > > > > kinds of vma. IOW, they want to reclaim anonymous pages by MADV_COLD > > > > > > > > while it keeps file-backed pages in inactive LRU by MADV_COOL because > > > > > > > > file IO is more expensive in this case so want to keep them in memory > > > > > > > > until memory pressure happens. > > > > > > > > > > > > > > > > To support such strategy easier, this patch introduces > > > > > > > > MADV_ANONYMOUS_FILTER and MADV_FILE_FILTER options in madvise(2) like > > > > > > > > that /proc/<pid>/clear_refs already has supported same filters. > > > > > > > > They are filters could be Ored with other existing hints using top two bits > > > > > > > > of (int behavior). > > > > > > > > > > > > > > madvise operates on top of ranges and it is quite trivial to do the > > > > > > > filtering from the userspace so why do we need any additional filtering? > > > > > > > > > > > > > > > Once either of them is set, the hint could affect only the interested vma > > > > > > > > either anonymous or file-backed. > > > > > > > > > > > > > > > > With that, user could call a process_madvise syscall simply with a entire > > > > > > > > range(0x0 - 0xFFFFFFFFFFFFFFFF) but either of MADV_ANONYMOUS_FILTER and > > > > > > > > MADV_FILE_FILTER so there is no need to call the syscall range by range. > > > > > > > > > > > > > > OK, so here is the reason you want that. The immediate question is why > > > > > > > cannot the monitor do the filtering from the userspace. Slightly more > > > > > > > work, all right, but less of an API to expose and that itself is a > > > > > > > strong argument against. > > > > > > > > > > > > What I should do if we don't have such filter option is to enumerate all of > > > > > > vma via /proc/<pid>/maps and then parse every ranges and inode from string, > > > > > > which would be painful for 2000+ vmas. > > > > > > > > > > Painful is not an argument to add a new user API. If the existing API > > > > > suits the purpose then it should be used. If it is not usable, we can > > > > > think of a different way. > > > > > > > > I measured 1568 vma parsing overhead of /proc/<pid>/maps in ARM64 modern > > > > mobile CPU. It takes 60ms and 185ms on big cores depending on cpu governor. > > > > It's never trivial. > > > > > > This is not the only option. Have you tried to simply use > > > /proc/<pid>/map_files interface? This will provide you with all the file > > > backed mappings. > > > > I compared maps vs. map_files with 3036 file-backed vma. > > Test scenario is to dump all of vmas of the process and parse address > > ranges. > > For map_files, it's easy to parse each address range because directory name > > itself is range. However, in case of maps, I need to parse each range > > line by line so need to scan all of lines. > > > > (maps cover additional non-file-backed vmas so nr_vma is a little bigger) > > > > performance mode: > > map_files: nr_vma 3036 usec 13387 > > maps : nr_vma 3078 usec 12923 > > > > powersave mode: > > > > map_files: nr_vma 3036 usec 52614 > > maps : nr_vma 3078 usec 41089 > > > > map_files is slower than maps if we dump all of vmas. I guess directory > > operation needs much more jobs(e.g., dentry lookup, instantiation) > > compared to maps. > > OK, that is somehow surprising. I am still not convinced the filter is a > good idea though. The primary reason is that it encourages using madvise > on a wide range without having a clue what the range contains. E.g. the > full address range and rely the right thing will happen. Do we really > want madvise to operate in that mode? If user space daemon(e.g., activity manager service) could know a certain process is bakground and idle for a while, yeb, that would be good option. > > Btw. if we went with the per vma fd approach then you would get this > feature automatically because map_files would refer to file backed > mappings while map_anon could refer only to anonymous mappings. The reason to add such filter option is to avoid the parsing overhead so map_anon wouldn't be helpful. > > -- > Michal Hocko > SUSE Labs
