Damien Zammit, le dim. 11 févr. 2024 11:30:31 +0000, a ecrit:
> On 2/11/24 10:07 PM, Samuel Thibault wrote:
> > Damien Zammit, le dim. 11 févr. 2024 10:55:26 +0000, a ecrit:
> >>>> diff --git a/kern/task.h b/kern/task.h
> >>>> index dec3a530..27970620 100644
> >>>> --- a/kern/task.h
> >>>> +++ b/kern/task.h
> >>>> @@ -61,11 +61,11 @@ struct task {
> >>>> decl_simple_lock_data(,lock) /* Task's lock */
> >>>> int ref_count; /* Number of references to me */
> >>>>
> >>>> - /* Flags */
> >>>> - unsigned int active:1, /* Task has not been terminated
> >>>> */
> >>>> - /* boolean_t */ may_assign:1, /* can assigned pset be
> >>>> changed? */
> >>>> - assign_active:1, /* waiting for
> >>>> may_assign */
> >>>> - essential:1; /* Is this task essential for
> >>>> the system? */
> >>>> + /* Addressable flags */
> >>>> + unsigned char active; /* Task has not been terminated
> >>>> */
> >>>> + unsigned char may_assign; /* can assigned pset be
> >>>> changed? */
> >>>> + unsigned char assign_active; /* waiting for may_assign */
> >>>> + unsigned char essential; /* Is this task essential for
> >>>> the system? */
> >>> AIUI only assign_active need to be adressable? Better make only that one
> >>> addressable.
> >> Looking at the existing flag types, it needs to fit into part of a
> >> cacheline.
> > Why would it need to fit in a cacheline?
>
> See comment above struct task:
>
> /*
> * Task name buffer size. The size is chosen so that struct task fits
> * into three cache lines. The size of a cache line on a typical CPU
> * is 64 bytes.
> */
> #define TASK_NAME_SIZE 32
Ok, but that's only for optimization.
> >> The way I have done it, I rearranged the existing 4 byte integer to be 4
> >> separate single byte flags.
> >> If you want me to put only one of the values into an addressable field,
> >> how will I retain the same size for the rest of the fields?
> > What do you mean by "the same size"?
>
> I think the actual sizeof(struct task) and its layout matters.
Ok
> Therefore, my original code change here preserves most of the layout and
> sizeof(struct task).
The original code uses 4 bits, so a byte. Your proposed change uses 4
bytes, so 3 more bytes. By using e.g.
unsigned char may_assign, /* can assigned pset be changed? */
unsigned int active:1, /* Task has not been terminated */
assign_active:1, /* waiting for may_assign */
essential:1; /* Is this task essential for the
system? */
That'll be 2 bytes, so even better for cache line size.
Samuel
