svn commit: r356656 - in head/sys: kern sys

[Mateusz Guzik]

On 1/12/20, Konstantin Belousov <kostikbel at gmail.com> wrote:
> On Sun, Jan 12, 2020 at 06:09:10AM +0000, Mateusz Guzik wrote:
>> Author: mjg
>> Date: Sun Jan 12 06:09:10 2020
>> New Revision: 356656
>> URL: https://svnweb.freebsd.org/changeset/base/356656
>>
>> Log:
>>   Add "panicked" boolean which can be tested instead of panicstr
>>
>>   The test is performed all the time and reading entire panicstr to do it
>>   wastes space.
> What space does it waste ?  The space in the read-mostly section ?
>
> And what is the difference between reading a pointer vs. reading a byte ?
> CPU still has to read the whole cache line.
>

Note the annotation with __read_frequently which groups fields.

Currently it results in the following (with tail filled up from
__read_mostly):

----------------
ffffffff80c01000 D hc_source_mask
ffffffff80c01004 D pti
ffffffff80c01008 D locks_delay
ffffffff80c0100c D locks_delay_retries
ffffffff80c0100e D locks_delay_loops
ffffffff80c01010 d fortuna_concurrent_read
ffffffff80c01011 D random_chachamode
ffffffff80c01012 D systrace_enabled
ffffffff80c01013 D lockstat_enabled
ffffffff80c01014 D dtrace_malloc_enabled
ffffffff80c01015 D racct_enable
ffffffff80c01016 D sdt_probes_enabled
ffffffff80c01017 D panicked
ffffffff80c01018 D kdb_active
ffffffff80c01019 D trap_enotcap
ffffffff80c0101a D audit_syscalls_enabled
ffffffff80c01020 D cam_dflags
ffffffff80c01028 d epoch_inited
ffffffff80c01030 d rs_epoch
ffffffff80c01038 d biozone
----------------

Should panicstr land here it would avoidably take up 8 bytes instead of 1.

-- 
Mateusz Guzik <mjguzik gmail.com>