Re: submit_bh leaves interrupts on upon return

[Jeff V. Merkey]

Jens Axboe wrote:

> On Wed, Jun 02 2004, Jeff V. Merkey wrote:
>  
>
> > Any reason why submit_bh should turn on interrupts after being called by 
> > a process with ints off in 2.4.20?  I see it's possible to sleep during 
> > elevatoring, but why does it need to leave interrupts on if the calling 
> > state was with ints off.  
> >    
>
> It's illegal to call it with interrupts off, so... __make_request()
> doesn't save interrupt state, so you will always leave with interrupts
> enabled.
>
>  
Jens

I noticed in the code it does not check for this when make_request is 
called, so I altered the calling sequence to call with ints on. I don't 
see much of a performance difference either way, so calling with ints on 
was easy to instrument. I am posting about 80,000+ buffer heads per 
second in with what I am doing, so filling out buffer_head structures 
and submitting them ad hoc was causing some interrupt windows where the 
chains were getting corrupted. I altered the calling sequence and added 
atomic counters so I can submit and call with ints on to avoid the 
corruption. One of the troublesome aspects of the manner in which 
make_request is implemented in always needing a context of a thread for 
sleeping to submit asynch I/O limits the ability to gang schedule large 
disk I/O from the b_end_io callback. Would make performance a lot more 
spectacular if it worked this way, but I am seeing good enough 
performance with it left the way it is. 3Ware's 66Mhz ATA adapter in 
this implementation is reaching almost 400 MB/S throughput on 2.4.20. I 
have not tried this on 2.6 yet, but will later this month.

Also, I ported the kernel debugger from MANOS to Linux and made a lot of 
significant enhancements and www.devicelogics.com is distributing it 
from their website. If anyone wants a more pleasant debugger for the 
kernel to work with, they are allowing downloads of the modules with a 
patch. Not free (what is in this world) but very nice to work with.

Thanks for the response. Sorry I was out of touch for about a year. I 
was going through a very nasty divorce with my wife of 24 years and I 
discovered when something like that is happening in your life, you don't 
have much attention for much else.

Jeff


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