On Tue, Sep 03, 2024 at 06:07:28PM GMT, Suren Baghdasaryan wrote: > On Sun, Sep 1, 2024 at 10:09 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > On Sun, 1 Sep 2024 21:41:27 -0700 Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > > > > Introduce CONFIG_PGALLOC_TAG_REF_BITS to control the size of the > > > page allocation tag references. When the size is configured to be > > > less than a direct pointer, the tags are searched using an index > > > stored as the tag reference. > > > > > > ... > > > > > > +config PGALLOC_TAG_REF_BITS > > > + int "Number of bits for page allocation tag reference (10-64)" > > > + range 10 64 > > > + default "64" > > > + depends on MEM_ALLOC_PROFILING > > > + help > > > + Number of bits used to encode a page allocation tag reference. > > > + > > > + Smaller number results in less memory overhead but limits the number of > > > + allocations which can be tagged (including allocations from modules). > > > + > > > > In other words, "we have no idea what's best for you, you're on your > > own". > > > > I pity our poor users. > > > > Can we at least tell them what they should look at to determine whether > > whatever random number they chose was helpful or harmful? > > At the end of my reply in > https://lore.kernel.org/all/CAJuCfpGNYgx0GW4suHRzmxVH28RGRnFBvFC6WO+F8BD4HDqxXA@xxxxxxxxxxxxxx/#t > I suggested using all unused page flags. That would simplify things > for the user at the expense of potentially using more memory than we > need. Why would that use more memory, and how much? > In practice 13 bits should be more than enough to cover all > kernel page allocations with enough headroom for page allocations > coming from loadable modules. I guess using 13 as the default would > cover most cases. In the unlikely case a specific system needs more > tags, the user can increase this value. It can also be set to 64 to > force direct references instead of indexing for better performance. > Would that approach be acceptable? Any knob that has to be kept track of and adjusted is a real hassle - e.g. lockdep has a bunch of knobs that have to be periodically tweaked, that's used by _developers_, and they're often wrong.
