* Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote: > On Thu, Oct 26, 2017 at 09:37:52AM +0200, Ingo Molnar wrote: > > > > * Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote: > > > > > On Tue, Oct 24, 2017 at 02:47:41PM +0200, Ingo Molnar wrote: > > > > > > > > > > Making a variable that 'looks' like a constant macro dynamic in a rare Kconfig > > > > > > > > > > scenario is asking for trouble. > > > > > > > > > > > > > > > > > > We expect boot-time page mode switching to be enabled in kernel of next > > > > > > > > > generation enterprise distros. It shoudn't be that rare. > > > > > > > > > > > > > > > > My point remains even with not-so-rare Kconfig dependency. > > > > > > > > > > > > > > I don't follow how introducing new variable that depends on Kconfig option > > > > > > > would help with the situation. > > > > > > > > > > > > A new, properly named variable or function (max_physmem_bits or > > > > > > max_physmem_bits()) that is not all uppercase would make it abundantly clear that > > > > > > it is not a constant but a runtime value. > > > > > > > > > > Would we need to rename every uppercase macros that would depend on > > > > > max_physmem_bits()? Like MAXMEM. > > > > > > > > MAXMEM isn't used in too many places either - what's the total impact of it? > > > > > > The impact is not very small. The tree of macros dependent on > > > MAX_PHYSMEM_BITS: > > > > > > MAX_PHYSMEM_BITS > > > MAXMEM > > > KEXEC_SOURCE_MEMORY_LIMIT > > > KEXEC_DESTINATION_MEMORY_LIMIT > > > KEXEC_CONTROL_MEMORY_LIMIT > > > SECTIONS_SHIFT > > > ZONEID_SHIFT > > > ZONEID_PGSHIFT > > > ZONEID_MASK > > > > > > The total number of users of them is not large. It's doable. But I expect > > > it to be somewhat ugly, since we're partly in generic code and it would > > > require some kind of compatibility layer for other archtectures. > > > > > > Do you want me to rename them all? > > > > Yeah, I think these former constants should be organized better. > > > > Here's their usage frequency: > > > > triton:~/tip> for N in MAX_PHYSMEM_BITS MAXMEM KEXEC_SOURCE_MEMORY_LIMIT \ > > KEXEC_DESTINATION_MEMORY_LIMIT KEXEC_CONTROL_MEMORY_LIMIT SECTIONS_SHIFT \ > > ZONEID_SHIFT ZONEID_PGSHIFT ZONEID_MASK; do printf " %-40s: " $N; git grep -w $N | grep -vE 'define| \* ' | wc -l; done > > > > MAX_PHYSMEM_BITS : 10 > > MAXMEM : 5 > > KEXEC_SOURCE_MEMORY_LIMIT : 2 > > KEXEC_DESTINATION_MEMORY_LIMIT : 2 > > KEXEC_CONTROL_MEMORY_LIMIT : 2 > > SECTIONS_SHIFT : 2 > > ZONEID_SHIFT : 1 > > ZONEID_PGSHIFT : 1 > > ZONEID_MASK : 1 > > > > So it's not too bad to clean up, I think. > > > > How about something like this: > > > > machine.physmem.max_bytes /* ex MAXMEM */ > > machine.physmem.max_bits /* bit count of the highest in-use physical address */ > > machine.physmem.zones.id_shift /* ZONEID_SHIFT */ > > machine.physmem.zones.pg_shift /* ZONEID_PGSHIFT */ > > machine.physmem.zones.id_mask /* ZONEID_MASK */ > > > > machine.kexec.physmem_bytes_src /* KEXEC_SOURCE_MEMORY_LIMIT */ > > machine.kexec.physmem_bytes_dst /* KEXEC_DESTINATION_MEMORY_LIMIT */ > > > > ( With perhaps 'physmem' being an alias to '&machine->physmem', so that > > physmem->max_bytes and physmem->max_bits would be a natural thing to write. ) > > > > I'd suggest doing this in a finegrained fashion, one step at a time, introducing > > 'struct machine' and 'struct physmem' and extending it gradually with new fields. > > I don't think this design is reasonable. > > - It introduces memory references where we haven't had them before. > > At this point all variable would fit a cache line, which is not that > bad. But I don't see what would stop the list from growing in the > future. Is any of these actually in a hotpath? Also, note the context: your changes turn some of these into variables. Yes, I suggest structuring them all and turning them all into variables, exactly because the majority are now dynamic, yet their _naming_ suggests that they are constants. > - We loose ability to optimize out change with static branches > (cpu_feature_enabled() instead of pgtable_l5_enabled variable). > > It's probably, not that big of an issue here, but if we are going to > use the same approach for other dynamic macros in the patchset, it > might be. Here too I think the (vast) majority of the uses here are for bootup/setup/init purposes, where clarity and maintainability of code matters a lot. > - AFAICS, it requires changes to all architectures to provide such > structures as we now partly in generic code. > > Or to introduce some kind of compatibility layer, but it would make > the kernel as a whole uglier than cleaner. Especially, given that > nobody beyond x86 need this. Yes, all the uses should be harmonized (no compatibility layer) - but as you can see it from the histogram I generated it's a few dozen uses, i.e. not too bad. Thanks, Ingo -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
