On Thu, Feb 18, 2021 at 11:24:19AM -0800, Yury Norov wrote: > On Thu, Feb 18, 2021 at 05:15:43PM +0200, Andy Shevchenko wrote: > > On Wed, Feb 17, 2021 at 08:05:06PM -0800, Yury Norov wrote: > > > This series introduces fast paths for find_bit() routines. It is > > > beneficial for typical systems, but those who limited in I-cache > > > may be concerned about increasing the .text size of the Image. > > > > > > To address this concern, one can disable FAST_PATH option in the config > > > and some save memory. > > > > > > The effect of this option on my arm64 next-20210217 build is: > > > > (Maybe bloat-o-meter will give better view on this, i.e. more human-readable) > > Never heard about this tool, thanks for the hint. > > scripts/bloat-o-meter vmlinux vmlinux.new > add/remove: 16/13 grow/shrink: 111/439 up/down: 3616/-19352 (-15736) > Function old new delta > find_next_bit.constprop - 220 +220 > apply_wqattrs_cleanup - 176 +176 > memcg_free_shrinker_maps - 172 +172 > ... > cpuset_hotplug_workfn 2584 2288 -296 > task_numa_fault 3640 3320 -320 > kmem_cache_free_bulk 1684 1280 -404 > Total: Before=26085140, After=26069404, chg -0.06% > > The complete output is here: > https://pastebin.com/kBSdVJcK > > So if I understand the output correctly, the size of .text is decreased... > Looks weird, but if it's true, we don't need the FAST_BIT config at all > because there's no tradeoff, and I should drop the patch. I actually expected the text size decrease when it's about constants. I remember that in PCI case we discussed with Bjorn the use of for_each_set_bit() that brought entire function into the object file that increased it by ~300 bytes (or so). But the code is something like for_each_set_bit(i, &addr, 32) ... > > I think the name is to broad for this cases, perhaps BITS_FAST_PATH? or BITMAP? > > My logic was that since SMALL_CONST() is global, and FAST_PATH > controls the SMALL_CONST, it should also be global. I believe, > Linux should have a global switch to control the behaviour in > such cases, similarly to -Os compiler option. And I was surprized > when I found nothing like FAST_PATH in the config. > > What about having FAST_PATH as a global option, and later if someone > will request for granularity, we'll introduce nested configs? I think it is too far from now. Let's do one step at a time. -- With Best Regards, Andy Shevchenko
