On 11/16/07, Mark Lord <liml@xxxxxx> wrote:
> Fajun Chen wrote:
> > Hi All,
> >
> > I use sg/libata and ata pass through for read/writes. Linux 2.6.18-rc2
> > and libata version 2.00 are loaded on ARM XScale board. Under heavy
> > cpu load (e.g. when blocks per transfer/sector count is set to 1),
> > I've observed that the test application can suck cpu away for long
> > time (more than 20 seconds) and other processes including high
> > priority shell can not get the time slice to run. What's interesting
> > is that if the application is under heavy IO load (e.g. when blocks
> > per transfer/sector count is set to 256), the problem goes away. I
> > also tested with open source code sg_utils and got the same result, so
> > this is not a problem specific to my user-space application.
> ..
>
> Post the relevant code here, and then we'll be able to better understand
> and explain it to you.
>
> For example, if the code is using ATA opcodes 0x20, 0x21, 0x24,
> 0x30, 0x31, 0x34, 0x29, 0x39, 0xc4 or 0xc5 (any of the R/W PIO ops),
> then this behaviour does not surprise me in the least. Fully expected
> and difficult to avoid.
>
This problem also happens with R/W DMA ops. Below are simplified code snippets:
// Open one sg device for read
if ((sg_fd = open(dev_name, O_RDWR))<0)
{
...
}
read_buffer = (U8 *)mmap(NULL, buf_sz, PROT_READ | PROT_WRITE,
MAP_SHARED, sg_fd, 0);
// Open the same sg device for write
if ((sg_fd_wr = open(dev_name, O_RDWR))<0)
{
...
}
write_buffer = (U8 *)mmap(NULL, buf_sz, PROT_READ | PROT_WRITE,
MAP_SHARED, sg_fd_wr, 0);
sg_io_hdr_t io_hdr;
memset(&io_hdr, 0, sizeof(sg_io_hdr_t));
io_hdr.interface_id = 'S';
io_hdr.mx_sb_len = sizeof(sense_buffer);
io_hdr.sbp = sense_buffer;
io_hdr.dxfer_len = dxfer_len;
io_hdr.cmd_len = cmd_len;
io_hdr.cmdp = cmdp; // ATA pass through command block
io_hdr.timeout = cmd_tmo * 1000; // In millisecs
io_hdr.pack_id = id; // Read/write counter for now
io_hdr.iovec_count=0; // scatter gather elements, 0=not being used
if (direction == 1)
{
io_hdr.dxfer_direction = SG_DXFER_TO_DEV;
io_hdr.flags |= SG_FLAG_MMAP_IO;
status = ioctl(sg_fd_wr, SG_IO, &io_hdr);
}
else
{
io_hdr.dxfer_direction = SG_DXFER_FROM_DEV;
io_hdr.flags |= SG_FLAG_MMAP_IO;
status = ioctl(sg_fd, SG_IO, &io_hdr);
}
...
Mmaped IO is a moot point here since this problem is also observed
when using direct IO.
Thanks,
Fajun
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