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    SubjectRe: [PATCH 1/7] dynamic debug v2 - infrastructure
    On Fri, Aug 08, 2008 at 06:07:41PM -0700, Greg KH wrote:
    > > Few notes...there is still one control file: <debugfs>/dynamic_printk/modules
    > > We can split this up now or later, but I kind of like being able to see all
    > > the controls in one file. Also, i've used a djb2 hash function in the code,
    > > which i'm not sure is under the correct license/copyright, so i just wanted
    > > to point that out as well. see: scripts/basic/hash.c. If its an issue, i can
    > > find another hash function.
    >
    > Why use that hash function? Actually, it looks like you are using 2
    > different ones, right? Why?
    >

    The hash functions are used to map between the calling module name,
    KBUILD_MODNAME, and whether or not debugging is enabled. In the 'fast' case a
    bitmask is used to represent each hash bucket. One bit per bucket. Thus, if
    all the modules in a bucket are disabled the bitmask is simply 0. Two
    hashes are used in order to reduce the probability of collisions, used in this
    way it is similar to a bloom filter. The code that implements this is:

    #define __dynamic_dbg_enabled(module, type, value, level, hash) ({ \
    int ret = 0; \
    if (unlikely((dynamic_printk_enabled & (1LL << DEBUG_HASH)) && \
    (dynamic_printk_enabled2 & (1LL << DEBUG_HASH2)))) \
    ret = __dynamic_dbg_enabled_helper(module, type, \
    value, hash);\
    ret; })

    where DEBUG_HASH and DEBUG_HASH2 are computed at compile time. Thus, in the
    'fast' case where debugging is off, we have two extra intructions, a test and
    a jmp. Further, if we enable debugging for a single module, we will at most
    do two 'tests' and a jump for disabled modules. If the module is enabled, we
    would call through to __dynamic_dbg_enabled_helper(), which verifies whether
    or not debugging is enabled for the module. I believe if we are then calling
    'printk()' the cost of this function call is 'in the noise'. Thus, i've tried
    to optimize the disabled case.

    > > --- /dev/null
    > > +++ b/include/linux/dynamic_printk.h
    > > @@ -0,0 +1,94 @@
    > > +#ifndef _DYNAMIC_PRINTK_H
    > > +#define _DYNAMIC_PRINTK_H
    > > +
    > > +#ifdef __KERNEL__
    > Shouldn't be needed.
    >

    ok

    > > +#include <linux/string.h>
    > > +#include <linux/hash.h>
    > > +
    > > +#define DYNAMIC_DEBUG_HASH_BITS 6
    > > +#define DEBUG_HASH_TABLE_SIZE (1 << DYNAMIC_DEBUG_HASH_BITS)
    > > +
    > > +#define TYPE_BOOLEAN 1
    >
    > What's this for?
    >

    This is really in anticipation of more 'involved' dynamic debugging. That is,
    'TYPE_BOOLEAN', just means the debugging is either off/on. The other types
    are 'TYPE_FLAG', where you can control debugging for a module via a bitmask,
    or 'TYPE_LEVEL', where you control the debugging for a module via a level
    control like printk levels. These different types were implemented in the
    orginal patchset.


    > > +#define DYNAMIC_ENABLED_ALL 0
    > > +#define DYNAMIC_ENABLED_NONE 1
    > > +#define DYNAMIC_ENABLED_SOME 2
    > > +
    > > +extern int dynamic_enabled;
    > > +extern long long dynamic_printk_enabled;
    > > +extern long long dynamic_printk_enabled2;
    >
    > What's up with the "2" versions?
    >

    "2" here is the second bitmask as explained previously.

    thanks,

    -Jason


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