Subject: [QUESTION,RFC] cacheable_memcpy() versus memcpy() ==> 8% improvment on FTP throughput

From: leroy christophe <[email-masked]>
Date: Wed, 11 Feb 2015 08:53:18 +0100

In powerpc32 architecture there is a function called cacheable_memcpy() 
which does same thing as memcpy() but using dcbz/dcbt instructions for 
an optimised copy (just like __copy_tofrom_user())
What seems strange is that it is almost nowhere used (only used in 
drivers/net/ethernet/ibm/emac/core.c)

For a try I replaced all memcpy() in include/linux/skbuff.h and 
net/core/skbuff.c by cacheable_memcpy() and I got around 8% improvement 
on FTP throughput on MPC885.

What could be done to generalise the use of cacheable_memcpy() instead 
of memcpy() whenever possible ?
Indeed, in order to use cacheable_memcpy(), we need
* The destination to be cacheable
* The source and destination to not overlap on the same cachelines

Could we check, when calling memcpy(), whether the destination is 
cacheable or not, and if yes redirect the call to cacheable_memcpy() ?
How can we check that ?

Christophe