On Fri, Aug 14, 2020 at 04:19:29AM +0100, Matthew Wilcox wrote: > On Fri, Aug 14, 2020 at 10:45:37AM +0800, Zhaoyang Huang wrote: > > On Fri, Aug 14, 2020 at 10:33 AM Andrew Morton > > <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > On Fri, 14 Aug 2020 10:20:11 +0800 Zhaoyang Huang <huangzhaoyang@xxxxxxxxx> wrote: > > > > > > > On Fri, Aug 14, 2020 at 10:07 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > > > > > > > On Fri, Aug 14, 2020 at 02:43:55AM +0100, Matthew Wilcox wrote: > > > > > > On Fri, Aug 14, 2020 at 09:30:11AM +0800, Zhaoyang Huang wrote: > > > > > > > file->f_ra->ra_pages will remain the initialized value since it opend, which may > > > > > > > be NOT equal to bdi->ra_pages as the latter one is updated somehow(etc, > > > > > > > echo xxx > /sys/block/dm/queue/read_ahead_kb).So sync ra->ra_pages to the > > > > > > > updated value when sync read. > > > > > > > > > > > > It still ignores the work done by shrink_readahead_size_eio() > > > > > > and fadvise(POSIX_FADV_SEQUENTIAL). > > > > > > > > > > ... by the way, if you're trying to update one particular file's readahead > > > > > state, you can just call fadvise(POSIX_FADV_NORMAL) on it. > > > > > > > > > > If you want to update every open file's ra_pages by writing to sysfs, > > > > > then just no. We don't do that. > > > > No, What I want to fix is the file within one process's context keeps > > > > using the initialized value when it is opened and not sync with new > > > > value when bdi->ra_pages changes. > > > > > > So you're saying that > > > > > > echo xxx > /sys/block/dm/queue/read_ahead_kb > > > > > > does not affect presently-open files, and you believe that it should do > > > so? > > > > > > I guess that could be a reasonable thing to want - it's reasonable for > > > a user to expect that writing to a global tunable will take immediate > > > global effect. I guess. > > > > > > But as Matthew says, it would help if you were to explain why this is > > > needed. In full detail. What operational problems is the present > > > implementation causing? > > The real scenario is some system(like android) will turbo read during > > startup via expanding the readahead window and then set it back to > > normal(128kb as usual). However, some files in the system process > > context will keep to be opened since it is opened up and has no chance > > to sync with the updated value as it is almost impossible to change > > the files attached to the inode(processes are unaware of these > > things). we have to fix it from a kernel perspective. > > OK, this is a much more useful description of the problem, thank you! It's not the first time we brought up the issue. https://patchwork.kernel.org/patch/10866161/ Hopefully, we have some solution at this time. > > I can think of two possibilities here. One is that maybe our readahead > heuristics just don't work on modern phone hardware. Perhaps we need > to ramp up more aggressively by default. > > The other is that maybe it really is just a "boost at startup" kind > of situation and so we should support _that_. Some interface where > we can set a ra_boost, and then do: > > if (ra_boost) > newsize *= 2; > > in get_init_ra_size(). With kernel boot paramter, it sounds good idea to me.
