As several people have pointed out, the metabolic requirements of a tribe of giants the size you want are hard to justify if you want them to rely on humans as a food source. It's not as bad as some would have it though, and it might be fixable if you bend biology a bit!
The giants are about 8 times taller than human males (source: observing my Scandinavian friends), and with the same proportions should weigh about 500 times as much. Let's say 40 tonnes.
According to Kleiber's law, food requirements scale roughly as an animal's metabolic rate scales approximately as mass to the power 3/4, and the giants thus each need to eat just about 100 times as much as a human.
How much human does a human need to eat to survive? According to What If:
"If the average human weighs 50 kilograms and eats a couple thousand calories per day, then—according to Ryan North—then one person contains enough meat to feed another person for about a month."
(My reference humans are heavier than 50 kilos, but not by THAT much, and if you would instead take the exponent in Kleiber's law to be 2/3 as some have argued this doesn't affect my calculations much.)
Using the scaling calculated above, we find that each giant would need about 3 people per day. If they could supplement their diet with animals or even some vegetables or grains, you could probably get down to one human a day and still have this qualify as a "major part of their diet".
10 giants would thus need about 3600 humans a year. This is still a lot, and places serious limitations on how an "uneducated" tribe of giants could look, if the word is meant to imply that they are like the dimwitted trope trolls we see in popular fiction.
As this question is marked with reality-check, I will answer "Yes, it's possible", but it's hard. I like the answer by Dario Quint, and if you only require the giants to snack on humans occasionally Willk's answer is really great!
If you are not strictly limited by our current understanding of biology, however, the answer changes to "Yes!". You just need a bit of currently un-understood biology, or "magic" if you will.
The first that came to my mind was to have the giants periodically turn to stone, like some trolls are known to do, but in a reversible way. People and animals made of stone are spread about many cities, and they are known to consume very little energy. If they depetrify during nights with a full moon, that's 60 percent of the time which translates to 6 humans a year per giant. They manage to catch on average one every other time they are awake, and otherwise they eat other animals. Of course you could have the depetrify even more rarely or more often, but keep in mind that the depetrification process itself likely requires some energy. If the cycle is repeated too often the gain will likely vanish.
If we assume the bowel content of the giants doesn't petrify, this opens up huge possibilities for them to use the long hibernation to digest stuff that is really hard for other creatures to use. Wood comes to mind, but grass or other plant matter works as well. You could thus have giants living i remote places, being able to sustain themselves largely on chewing up trees (or even houses, as I'm sure I've read about giant's doing!) and stealing cattle, while sometimes catching humans as an important supplement to their diet!
Long ago, giants were more numerous than today and constituted a small but reproductively stable population. Early in human history, one of the early projects of the emerging civilisations was to seek out and destroy giants, mostly while they were petrified. This is why you find legends about giants everywhere.
Since their lifespans are so stretched out, even a declining and not reproductively stable population can linger for a very long time. Over the last few thousand years, giants have learned to be extremely cautious around humans and to avoid detection at all costs. One main reason for them to prefer eating humans to other animals is simple revenge. Most of them hate humans above anything else!
The petrification process is precisely controlled by the giant biology, to make sure the stone is both resilient and reversible. Dead giants petrify in a chaotic manner that leaves the stone brittle, and are not readily distinguishable as humanoid after only a few years or even months. This is why we don't find any giant remains that would expose them to the world. Any proof of their existence are lost to history.