Re: [PATCH v2 3/3] scsi/ncr5380: Improve interrupt latency during PIO tranfers
> "Finn" == Finn Thain writes: Finn> Large PIO transfers are broken up into chunks to try to avoid Finn> disabling local IRQs for long periods. But IRQs are still disabled Finn> for too long and this causes SCC FIFO overruns during serial port Finn> transfers. Finn> This patch reduces the PIO chunk size to reduce interrupt latency Finn> to something on the order of milliseconds, at the expense of Finn> additional CPU overhead from extra iterations of the Finn> NCR5380_main() loop. Applied to 4.9/scsi-queue. -- Martin K. Petersen Oracle Linux Engineering
Re: [PATCH v2 3/3] scsi/ncr5380: Improve interrupt latency during PIO tranfers
On Fri, Sep 9, 2016 at 1:28 PM, Martin K. Petersen
wrote:
>> "Finn" == Finn Thain writes:
>
> Finn> Large PIO transfers are broken up into chunks to try to avoid
> Finn> disabling local IRQs for long periods. But IRQs are still disabled
> Finn> for too long and this causes SCC FIFO overruns during serial port
> Finn> transfers.
>
> Finn> This patch reduces the PIO chunk size to reduce interrupt latency
> Finn> to something on the order of milliseconds, at the expense of
> Finn> additional CPU overhead from extra iterations of the
> Finn> NCR5380_main() loop.
>
> Finn> That CPU overhead is a problem for slow machines (e.g. mac_scsi on
> Finn> 25 MHz
> Finn> 68030) but these machines generally use PDMA not PIO. This patch
> Finn>doesn't
> Finn> make the overhead any worse on my Mac LC III (because it only gets
> Finn> about 510 accesses per ms).
>
> Finn> This patch decreases disk performance by a fraction of one percent
> Finn> for dmx3191d on my 333 MHz PowerPC 750. Other affected hardware
> Finn> (such as g_NCR5380 on x86) was not tested but 5380 ISA cards
> Finn> generally use PDMA and not PIO.
>
> Geert?
No comments from my side.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- ge...@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
Re: [PATCH v2 3/3] scsi/ncr5380: Improve interrupt latency during PIO tranfers
> "Finn" == Finn Thain writes: Finn> Large PIO transfers are broken up into chunks to try to avoid Finn> disabling local IRQs for long periods. But IRQs are still disabled Finn> for too long and this causes SCC FIFO overruns during serial port Finn> transfers. Finn> This patch reduces the PIO chunk size to reduce interrupt latency Finn> to something on the order of milliseconds, at the expense of Finn> additional CPU overhead from extra iterations of the Finn> NCR5380_main() loop. Finn> That CPU overhead is a problem for slow machines (e.g. mac_scsi on Finn> 25 MHz Finn> 68030) but these machines generally use PDMA not PIO. This patch Finn>doesn't Finn> make the overhead any worse on my Mac LC III (because it only gets Finn> about 510 accesses per ms). Finn> This patch decreases disk performance by a fraction of one percent Finn> for dmx3191d on my 333 MHz PowerPC 750. Other affected hardware Finn> (such as g_NCR5380 on x86) was not tested but 5380 ISA cards Finn> generally use PDMA and not PIO. Geert? -- Martin K. Petersen Oracle Linux Engineering
Re: [PATCH v2 3/3] scsi/ncr5380: Improve interrupt latency during PIO tranfers
- Original Message -
> From: "Finn Thain"
> To: "James E.J. Bottomley" , "Martin K. Petersen"
> , "Geert
> Uytterhoeven" , linux-s...@vger.kernel.org,
> linux-m...@vger.kernel.org,
> linux-kernel@vger.kernel.org, "Michael Schmitz"
> Sent: Wednesday, August 31, 2016 12:44:56 AM
> Subject: [PATCH v2 3/3] scsi/ncr5380: Improve interrupt latency during PIO
> tranfers
>
> Large PIO transfers are broken up into chunks to try to avoid disabling
> local IRQs for long periods. But IRQs are still disabled for too long
> and this causes SCC FIFO overruns during serial port transfers.
>
> This patch reduces the PIO chunk size to reduce interrupt latency to
> something on the order of milliseconds, at the expense of additional CPU
> overhead from extra iterations of the NCR5380_main() loop.
>
> That CPU overhead is a problem for slow machines (e.g. mac_scsi on 25 MHz
> 68030) but these machines generally use PDMA not PIO. This patch doesn't
> make the overhead any worse on my Mac LC III (because it only gets about
> 510 accesses per ms).
>
> This patch decreases disk performance by a fraction of one percent for
> dmx3191d on my 333 MHz PowerPC 750. Other affected hardware (such as
> g_NCR5380 on x86) was not tested but 5380 ISA cards generally use PDMA
> and not PIO.
>
> Signed-off-by: Finn Thain
>
> ---
> Changed since v1:
> - PIO transfer chunk size is now hard-coded for simplicity.
>
> ---
> drivers/scsi/NCR5380.c |8
> drivers/scsi/NCR5380.h |2 ++
> 2 files changed, 6 insertions(+), 4 deletions(-)
>
> Index: linux/drivers/scsi/NCR5380.c
> ===
> --- linux.orig/drivers/scsi/NCR5380.c 2016-08-31 14:44:51.0 +1000
> +++ linux/drivers/scsi/NCR5380.c 2016-08-31 14:44:52.0 +1000
> @@ -1849,11 +1849,11 @@ static void NCR5380_information_transfer
> /* XXX - need to source or sink
> data here, as appropriate */
> }
> } else {
> - /* Break up transfer into 3 ms chunks,
> - * presuming 6 accesses per handshake.
> + /* Transfer a small chunk so that the
> + * irq mode lock is not held too long.
>*/
> - transfersize = min((unsigned
> long)cmd->SCp.this_residual,
> -
> hostdata->accesses_per_ms / 2);
> + transfersize =
> min(cmd->SCp.this_residual,
> +
> NCR5380_PIO_CHUNK_SIZE);
> len = transfersize;
> NCR5380_transfer_pio(instance, &phase,
> &len,
>(unsigned char
> **)&cmd->SCp.ptr);
> Index: linux/drivers/scsi/NCR5380.h
> ===
> --- linux.orig/drivers/scsi/NCR5380.h 2016-08-31 14:44:51.0 +1000
> +++ linux/drivers/scsi/NCR5380.h 2016-08-31 14:44:52.0 +1000
> @@ -250,6 +250,8 @@ struct NCR5380_cmd {
>
> #define NCR5380_CMD_SIZE (sizeof(struct NCR5380_cmd))
>
> +#define NCR5380_PIO_CHUNK_SIZE 256
> +
> static inline struct scsi_cmnd *NCR5380_to_scmd(struct NCR5380_cmd
> *ncmd_ptr)
> {
> return ((struct scsi_cmnd *)ncmd_ptr) - 1;
>
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-scsi" in
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> More majordomo info at http://vger.kernel.org/majordomo-info.html
>
Patch makes sense, look good to me.
Reviwed-by: Laurence Oberman
[PATCH v2 3/3] scsi/ncr5380: Improve interrupt latency during PIO tranfers
Large PIO transfers are broken up into chunks to try to avoid disabling
local IRQs for long periods. But IRQs are still disabled for too long
and this causes SCC FIFO overruns during serial port transfers.
This patch reduces the PIO chunk size to reduce interrupt latency to
something on the order of milliseconds, at the expense of additional CPU
overhead from extra iterations of the NCR5380_main() loop.
That CPU overhead is a problem for slow machines (e.g. mac_scsi on 25 MHz
68030) but these machines generally use PDMA not PIO. This patch doesn't
make the overhead any worse on my Mac LC III (because it only gets about
510 accesses per ms).
This patch decreases disk performance by a fraction of one percent for
dmx3191d on my 333 MHz PowerPC 750. Other affected hardware (such as
g_NCR5380 on x86) was not tested but 5380 ISA cards generally use PDMA
and not PIO.
Signed-off-by: Finn Thain
---
Changed since v1:
- PIO transfer chunk size is now hard-coded for simplicity.
---
drivers/scsi/NCR5380.c |8
drivers/scsi/NCR5380.h |2 ++
2 files changed, 6 insertions(+), 4 deletions(-)
Index: linux/drivers/scsi/NCR5380.c
===
--- linux.orig/drivers/scsi/NCR5380.c 2016-08-31 14:44:51.0 +1000
+++ linux/drivers/scsi/NCR5380.c2016-08-31 14:44:52.0 +1000
@@ -1849,11 +1849,11 @@ static void NCR5380_information_transfer
/* XXX - need to source or sink
data here, as appropriate */
}
} else {
- /* Break up transfer into 3 ms chunks,
-* presuming 6 accesses per handshake.
+ /* Transfer a small chunk so that the
+* irq mode lock is not held too long.
*/
- transfersize = min((unsigned
long)cmd->SCp.this_residual,
-
hostdata->accesses_per_ms / 2);
+ transfersize =
min(cmd->SCp.this_residual,
+
NCR5380_PIO_CHUNK_SIZE);
len = transfersize;
NCR5380_transfer_pio(instance, &phase,
&len,
(unsigned char
**)&cmd->SCp.ptr);
Index: linux/drivers/scsi/NCR5380.h
===
--- linux.orig/drivers/scsi/NCR5380.h 2016-08-31 14:44:51.0 +1000
+++ linux/drivers/scsi/NCR5380.h2016-08-31 14:44:52.0 +1000
@@ -250,6 +250,8 @@ struct NCR5380_cmd {
#define NCR5380_CMD_SIZE (sizeof(struct NCR5380_cmd))
+#define NCR5380_PIO_CHUNK_SIZE 256
+
static inline struct scsi_cmnd *NCR5380_to_scmd(struct NCR5380_cmd *ncmd_ptr)
{
return ((struct scsi_cmnd *)ncmd_ptr) - 1;
