V3 [PATCH] libc.so: Add --list-tunables support to __libc_main

[H.J. Lu]

On Fri, Jun 5, 2020 at 3:45 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Thu, Jun 04, 2020 at 02:00:35PM -0700, H.J. Lu wrote:
> > On Mon, Jun 1, 2020 at 7:08 PM Carlos O'Donell <carlos@redhat.com> wrote:
> > >
> > > On Mon, Jun 1, 2020 at 6:44 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> > > > Tunables are designed to pass info from user to glibc, not the other
> > > > way around.  When __libc_main is called, init_cacheinfo is never
> > > > called.  I can call init_cacheinfo from __libc_main.  But there is no
> > > > interface to update min and max values from init_cacheinfo.  I don't
> > > > think --list-tunables will work here without changes to tunables.
> > >
> > > You have a dynamic threshold.
> > >
> > > You have to tell the user what that minimum is, otherwise they can't
> > > use the tunable reliably.
> > >
> > > This is the first instance of a min/max that is dynamically determined.
> > >
> > > You must fetch the cache info ahead of the tunable initialization, that
> > > is you must call init_cacheinfo before __init_tunables.
> > >
> > > You can initialize the tunable data dynamically like this:
> > >
> > > /* Dynamically set the min and max of glibc.foo.bar.  */
> > > tunable_id_t id = TUNABLE_ENUM_NAME (glibc, foo, bar);
> > > tunable_list[id].type.min = lowval;
> > > tunable_list[id].type.max = highval;
> > >
> > > We do something similar for maybe_enable_malloc_check.
> > >
> > > Then once the tunables are parsed, and the cpu features are loaded
> > > you can print the tunables, and the printed tunables will have meaningful
> > > min and max values.
> > >
> > > If you have circular dependency, then you must process the cpu features
> > > first without reading from the tunables, then allow the tunables to be
> > > initialized from the system, *then* process the tunables to alter the existing
> > > cpu feature settings.
> > >
> >
> > How about this?  I got
> >
>
> Here is the updated patch, which depends on
>
> https://sourceware.org/pipermail/libc-alpha/2020-June/114820.html
>
> to add "%d" support to _dl_debug_vdprintf.  I got
>
> $ ./elf/ld.so ./libc.so --list-tunables
> glibc.elision.skip_lock_after_retries: 3 (min: -2147483648, max: 2147483647)
> glibc.malloc.trim_threshold: 0x0 (min: 0x0, max: 0xffffffff)
> glibc.malloc.perturb: 0 (min: 0, max: 255)
> glibc.cpu.x86_shared_cache_size: 0x100000 (min: 0x0, max: 0xffffffff)
> glibc.elision.tries: 3 (min: -2147483648, max: 2147483647)
> glibc.elision.enable: 0 (min: 0, max: 1)
> glibc.malloc.mxfast: 0x0 (min: 0x0, max: 0xffffffff)
> glibc.elision.skip_lock_busy: 3 (min: -2147483648, max: 2147483647)
> glibc.malloc.top_pad: 0x0 (min: 0x0, max: 0xffffffff)
> glibc.cpu.x86_non_temporal_threshold: 0x600000 (min: 0x0, max: 0xffffffff)
> glibc.cpu.x86_shstk:
> glibc.cpu.hwcap_mask: 0x6 (min: 0x0, max: 0xffffffff)
> glibc.malloc.mmap_max: 0 (min: -2147483648, max: 2147483647)
> glibc.elision.skip_trylock_internal_abort: 3 (min: -2147483648, max: 2147483647)
> glibc.malloc.tcache_unsorted_limit: 0x0 (min: 0x0, max: 0xffffffff)
> glibc.cpu.x86_ibt:
> glibc.cpu.hwcaps:
> glibc.elision.skip_lock_internal_abort: 3 (min: -2147483648, max: 2147483647)
> glibc.malloc.arena_max: 0x0 (min: 0x1, max: 0xffffffff)
> glibc.malloc.mmap_threshold: 0x0 (min: 0x0, max: 0xffffffff)
> glibc.cpu.x86_data_cache_size: 0x8000 (min: 0x0, max: 0xffffffff)
> glibc.malloc.tcache_count: 0x0 (min: 0x0, max: 0xffffffff)
> glibc.malloc.arena_test: 0x0 (min: 0x1, max: 0xffffffff)
> glibc.pthread.mutex_spin_count: 100 (min: 0, max: 32767)
> glibc.malloc.tcache_max: 0x0 (min: 0x0, max: 0xffffffff)
> glibc.malloc.check: 0 (min: 0, max: 3)
> $
>
> Ok for master?
>

Here is the updated patch.  To support --list-tunables, a target should add

CPPFLAGS-version.c = -DLIBC_MAIN=__libc_main_body
CPPFLAGS-libc-main.S = -DLIBC_MAIN=__libc_main_body

and start.S should be updated to define __libc_main and call
__libc_main_body:

extern void __libc_main_body (int argc, char **argv)
  __attribute__ ((noreturn, visibility ("hidden")));

when LIBC_MAIN is defined.


--
H.J.
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