Al Viro writes: > On Tue, Jul 08, 2014 at 08:32:55PM +0100, Peter Maydell wrote: > On 8 July 2014 18:20, Al Viro <v...@zeniv.linux.org.uk> wrote: >> > On Tue, Jul 08, 2014 at 05:33:16PM +0100, Peter Maydell wrote: <snip> > Again, gprof isn't particulary useful - kernel-side profilers are at least as > good. So I suspect that most of the people running into that simply shrug and > use those instead. Narrowing it down to -pie didn't take long and I can > confirm that this is the root cause of that breakage. Should make debugging > said toolchain bug a bit easier, if anybody cares to do that... > >> > Stats I quoted were from qemu-system-alpha booting debian/lenny (5.10) and >> > going through their kernel package build. I have perf report in front of >> > me right now; the top ones are >> > 41.77% qemu-system-alp perf-24701.map [.] 0x7fbbee558930 >> > 11.78% qemu-system-alp qemu-system-alpha [.] cpu_alpha_exec >> >> > and cpu_alpha_exec() spends most of the time in inlined tb_find_fast(). >> > It might be worth checking the actual distribution of the hash of virt >> > address used by that sucker - I wonder if dividing its argument by 4 >> > wouldn't improve the things, but I don't have stats on actual frequency >> > of conflicts, etc. In any case, the first lump (42%) seems to be tastier >> > ;-) >> >> Depends on your point of view -- arguably we ought to be spending *more* >> time executing translated guest code... (As you say, the problem is that >> we don't have any breakdown of what things might turn out to be hotspots >> in the translated code.) > > Might be a fun project to teach perf that hits in such-and-such page should > lead to lookup in a table annotating it. As in "offsets 42..69 should be > recorded as (<this address> + offset - 42). Then tcg could generate > such tables and we'd get information like "that much time is spent in > the second host insn of instances of that code pattern generated by > tcg_gen_shr_i64", etc. > > No idea if anything of that sort exists - qemu is not the only possible user > for that; looks like it might be useful for any JIT profiling, so somebody > could've done that already...
Handily our patch tracker has remembered what I couldn't find ;-) https://patches.linaro.org/27229/ As I mentioned previously I plan to clean these up over the next week. -- Alex Bennée
